feat(auth): Microsoft OAuth sign-in/link + self-serve signup via Google/Microsoft

Wave 7. Mirrors the existing Google ID-token-exchange pattern (Program.cs
smart-scheme dispatch, JWT bearer scheme, AuthController exchange/link/
unlink endpoints, ApplicationUser fields, reconciler columns) for
Microsoft Entra ID + personal accounts via the multi-tenant "common"
endpoint.

Google/Microsoft sign-in previously only worked for accounts already
linked to an existing local user -- there was no way to actually sign
up via OAuth. Both exchange endpoints now create a new user when no
match is found and Auth:AllowRegistration is true, same gate as
email/password registration.

Frontend: new MicrosoftAuthCard (MSAL popup flow -- Microsoft has no
vanilla-JS equivalent to Google's Identity Services script) wired into
the login page's provider tabs and the profile page's account-linking
section. REACT_APP_MICROSOFT_CLIENT_ID env var, Auth:MicrosoftClientId
config gate on the backend.

JobTrackerApi/Controllers/AuthController.cs

@@ -19,15 +19,17 @@ public sealed class AuthController : ControllerBase
     private readonly ITokenService _tokens;
     private readonly IAppEmailSender _email;
     private readonly IGoogleTokenValidator _googleTokens;
+    private readonly IMicrosoftTokenValidator _microsoftTokens;
     private readonly ILogger<AuthController> _logger;
 
-    public AuthController(IConfiguration cfg, UserManager<ApplicationUser> users, ITokenService tokens, IAppEmailSender email, IGoogleTokenValidator googleTokens, ILogger<AuthController> logger)
+    public AuthController(IConfiguration cfg, UserManager<ApplicationUser> users, ITokenService tokens, IAppEmailSender email, IGoogleTokenValidator googleTokens, IMicrosoftTokenValidator microsoftTokens, ILogger<AuthController> logger)
     {
         _cfg = cfg;
         _users = users;
         _tokens = tokens;
         _email = email;
         _googleTokens = googleTokens;
+        _microsoftTokens = microsoftTokens;
         _logger = logger;
     }
 
@@ -37,12 +39,14 @@ public sealed class AuthController : ControllerBase
     {
         var requireAuth = _cfg.GetValue("Auth:Require", false);
         var googleEnabled = !string.IsNullOrWhiteSpace((_cfg["Auth:GoogleClientId"] ?? string.Empty).Trim());
+        var microsoftEnabled = !string.IsNullOrWhiteSpace((_cfg["Auth:MicrosoftClientId"] ?? string.Empty).Trim());
         var allowRegistration = _cfg.GetValue("Auth:AllowRegistration", false);
 
         return Ok(new
         {
             requireAuth,
             googleEnabled,
+            microsoftEnabled,
             localEnabled = true,
             allowRegistration,
         });
@@ -52,6 +56,7 @@ public sealed class AuthController : ControllerBase
     public sealed record RegisterRequest(string Email, string Password, bool RememberMe = true);
     public sealed record AuthSessionResult(bool Authenticated, string Provider);
     public sealed record GoogleLinkDto(bool Linked, string? Email, DateTimeOffset? LinkedAt);
+    public sealed record MicrosoftLinkDto(bool Linked, string? Email, DateTimeOffset? LinkedAt);
     public sealed record MeResult(
         string Provider,
         string? Id,
@@ -64,7 +69,8 @@ public sealed class AuthController : ControllerBase
         string? ProfileCvStructureJson,
         string? AvatarImageDataUrl,
         IList<string> Roles,
-        GoogleLinkDto? GoogleLink);
+        GoogleLinkDto? GoogleLink,
+        MicrosoftLinkDto? MicrosoftLink);
     private const int MaxAvatarBytes = 1_000_000;
     private static readonly HashSet<string> AllowedAvatarExtensions = new(StringComparer.OrdinalIgnoreCase)
     {
@@ -72,6 +78,7 @@ public sealed class AuthController : ControllerBase
     };
     public sealed record UpdateProfileRequest(string? Email, string? UserName, string? FirstName, string? LastName, string? DisplayName, string? ProfileCvText, string? ProfileCvStructureJson);
     public sealed record GoogleTokenRequest(string Token, bool RememberMe = true);
+    public sealed record MicrosoftTokenRequest(string Token, bool RememberMe = true);
 
     [HttpPost("login")]
     [AllowAnonymous]
@@ -155,7 +162,24 @@ public sealed class AuthController : ControllerBase
 
         if (user is null)
         {
-            return Unauthorized("This Google account is not linked to a Jobbjakt user yet.");
+            if (!google.EmailVerified || string.IsNullOrWhiteSpace(google.Email))
+            {
+                return Unauthorized("This Google account is not linked to a Jobbjakt user yet.");
+            }
+
+            var allowRegistration = _cfg.GetValue("Auth:AllowRegistration", false);
+            if (!allowRegistration)
+            {
+                return Unauthorized("This Google account is not linked to a Jobbjakt user yet.");
+            }
+
+            user = new ApplicationUser { UserName = google.Email, Email = google.Email, EmailConfirmed = true };
+            var created = await _users.CreateAsync(user);
+            if (!created.Succeeded)
+            {
+                return BadRequest(string.Join("; ", created.Errors.Select(e => e.Description)));
+            }
+            _logger.LogInformation("Created new user via Google sign-up for {Email}", google.Email);
         }
 
         if (string.IsNullOrWhiteSpace(user.GoogleSubject) || !string.Equals(user.GoogleSubject, google.Subject, StringComparison.Ordinal))
@@ -173,6 +197,74 @@ public sealed class AuthController : ControllerBase
         return Ok(new AuthSessionResult(true, "google"));
     }
 
+    [HttpPost("microsoft/exchange")]
+    [AllowAnonymous]
+    [EnableRateLimiting("auth-login")]
+    public async Task<ActionResult<AuthSessionResult>> ExchangeMicrosoftToken([FromBody] MicrosoftTokenRequest request, CancellationToken cancellationToken)
+    {
+        var token = (request.Token ?? string.Empty).Trim();
+        if (token.Length == 0) return BadRequest("Microsoft token is required.");
+
+        MicrosoftTokenPrincipal microsoft;
+        try
+        {
+            microsoft = await _microsoftTokens.ValidateAsync(token, cancellationToken);
+        }
+        catch (Exception ex)
+        {
+            return Unauthorized(ex.Message);
+        }
+
+        var user = await _users.Users.FirstOrDefaultAsync(
+            x => x.MicrosoftSubject == microsoft.Subject || (!string.IsNullOrWhiteSpace(microsoft.Email) && x.MicrosoftEmail == microsoft.Email),
+            cancellationToken);
+
+        if (user is null && microsoft.EmailVerified && !string.IsNullOrWhiteSpace(microsoft.Email))
+        {
+            user = await _users.FindByEmailAsync(microsoft.Email);
+            if (user is not null)
+            {
+                _logger.LogInformation("Auto-linking Microsoft sign-in for existing local account {Email}", microsoft.Email);
+            }
+        }
+
+        if (user is null)
+        {
+            if (!microsoft.EmailVerified || string.IsNullOrWhiteSpace(microsoft.Email))
+            {
+                return Unauthorized("This Microsoft account is not linked to a Jobbjakt user yet.");
+            }
+
+            var allowRegistration = _cfg.GetValue("Auth:AllowRegistration", false);
+            if (!allowRegistration)
+            {
+                return Unauthorized("This Microsoft account is not linked to a Jobbjakt user yet.");
+            }
+
+            user = new ApplicationUser { UserName = microsoft.Email, Email = microsoft.Email, EmailConfirmed = true };
+            var created = await _users.CreateAsync(user);
+            if (!created.Succeeded)
+            {
+                return BadRequest(string.Join("; ", created.Errors.Select(e => e.Description)));
+            }
+            _logger.LogInformation("Created new user via Microsoft sign-up for {Email}", microsoft.Email);
+        }
+
+        if (string.IsNullOrWhiteSpace(user.MicrosoftSubject) || !string.Equals(user.MicrosoftSubject, microsoft.Subject, StringComparison.Ordinal))
+        {
+            user.MicrosoftSubject = microsoft.Subject;
+            user.MicrosoftEmail = microsoft.Email;
+            user.MicrosoftLinkedAt ??= DateTimeOffset.UtcNow;
+            user.DisplayName ??= TrimOrNull(microsoft.Name);
+            user.FirstName ??= TrimOrNull(microsoft.GivenName);
+            user.LastName ??= TrimOrNull(microsoft.FamilyName);
+            await _users.UpdateAsync(user);
+        }
+
+        await SignInWithAppSessionAsync(user, request.RememberMe, cancellationToken);
+        return Ok(new AuthSessionResult(true, "microsoft"));
+    }
+
     [HttpPost("logout")]
     public IActionResult Logout()
     {
@@ -202,7 +294,11 @@ public sealed class AuthController : ControllerBase
         var email = User.FindFirstValue(ClaimTypes.Email) ?? User.FindFirstValue("email");
         var sub = User.FindFirstValue(ClaimTypes.NameIdentifier) ?? User.FindFirstValue("sub");
         var iss = User.FindFirstValue("iss") ?? string.Empty;
-        var provider = iss.Contains("accounts.google.com", StringComparison.OrdinalIgnoreCase) ? "google" : "external";
+        var provider = iss.Contains("accounts.google.com", StringComparison.OrdinalIgnoreCase)
+            ? "google"
+            : iss.Contains("login.microsoftonline.com", StringComparison.OrdinalIgnoreCase)
+                ? "microsoft"
+                : "external";
 
         return Ok(new MeResult(
             Provider: provider,
@@ -216,7 +312,8 @@ public sealed class AuthController : ControllerBase
             ProfileCvStructureJson: null,
             AvatarImageDataUrl: null,
             Roles: Array.Empty<string>(),
-            GoogleLink: provider == "google" ? new GoogleLinkDto(false, email, null) : null));
+            GoogleLink: provider == "google" ? new GoogleLinkDto(false, email, null) : null,
+            MicrosoftLink: provider == "microsoft" ? new MicrosoftLinkDto(false, email, null) : null));
     }
 
     [HttpPut("profile")]
@@ -322,6 +419,76 @@ public sealed class AuthController : ControllerBase
         return NoContent();
     }
 
+    [HttpPost("microsoft/link")]
+    [Authorize(AuthenticationSchemes = "local")]
+    public async Task<ActionResult<MicrosoftLinkDto>> LinkMicrosoft([FromBody] MicrosoftTokenRequest request, CancellationToken cancellationToken)
+    {
+        var user = await _users.GetUserAsync(User);
+        if (user is null)
+        {
+            return Unauthorized();
+        }
+
+        var token = (request.Token ?? string.Empty).Trim();
+        if (token.Length == 0) return BadRequest("Microsoft token is required.");
+
+        MicrosoftTokenPrincipal microsoft;
+        try
+        {
+            microsoft = await _microsoftTokens.ValidateAsync(token, cancellationToken);
+        }
+        catch (Exception ex)
+        {
+            return BadRequest(ex.Message);
+        }
+
+        var conflict = await _users.Users
+            .Where(x => x.Id != user.Id)
+            .FirstOrDefaultAsync(x => x.MicrosoftSubject == microsoft.Subject || (!string.IsNullOrWhiteSpace(microsoft.Email) && x.MicrosoftEmail == microsoft.Email), cancellationToken);
+        if (conflict is not null)
+        {
+            return Conflict("That Microsoft account is already linked to another Jobbjakt user.");
+        }
+
+        user.MicrosoftSubject = microsoft.Subject;
+        user.MicrosoftEmail = microsoft.Email;
+        user.MicrosoftLinkedAt = DateTimeOffset.UtcNow;
+        user.DisplayName ??= TrimOrNull(microsoft.Name);
+        user.FirstName ??= TrimOrNull(microsoft.GivenName);
+        user.LastName ??= TrimOrNull(microsoft.FamilyName);
+
+        var result = await _users.UpdateAsync(user);
+        if (!result.Succeeded)
+        {
+            return BadRequest(string.Join("; ", result.Errors.Select(e => e.Description)));
+        }
+
+        return Ok(new MicrosoftLinkDto(true, user.MicrosoftEmail, user.MicrosoftLinkedAt));
+    }
+
+    [HttpDelete("microsoft/link")]
+    [Authorize(AuthenticationSchemes = "local")]
+    public async Task<IActionResult> UnlinkMicrosoft()
+    {
+        var user = await _users.GetUserAsync(User);
+        if (user is null)
+        {
+            return Unauthorized();
+        }
+
+        user.MicrosoftSubject = null;
+        user.MicrosoftEmail = null;
+        user.MicrosoftLinkedAt = null;
+
+        var result = await _users.UpdateAsync(user);
+        if (!result.Succeeded)
+        {
+            return BadRequest(string.Join("; ", result.Errors.Select(e => e.Description)));
+        }
+
+        return NoContent();
+    }
+
     [HttpPost("avatar")]
     [Authorize(AuthenticationSchemes = "local")]
     [RequestSizeLimit(MaxAvatarBytes)]
@@ -571,6 +738,10 @@ public sealed class AuthController : ControllerBase
             GoogleLink: new GoogleLinkDto(
                 Linked: !string.IsNullOrWhiteSpace(user.GoogleSubject),
                 Email: user.GoogleEmail,
-                LinkedAt: user.GoogleLinkedAt));
+                LinkedAt: user.GoogleLinkedAt),
+            MicrosoftLink: new MicrosoftLinkDto(
+                Linked: !string.IsNullOrWhiteSpace(user.MicrosoftSubject),
+                Email: user.MicrosoftEmail,
+                LinkedAt: user.MicrosoftLinkedAt));
     }
 }