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.Tests/AuthAndSystemControllerTests.cs

@@ -25,7 +25,7 @@ public sealed class AuthAndSystemControllerTests
         userManager.Setup(x => x.GetUserAsync(It.IsAny<System.Security.Claims.ClaimsPrincipal>())).ReturnsAsync(user);
         userManager.Setup(x => x.UpdateAsync(user)).ReturnsAsync(IdentityResult.Success);
 
-        var controller = new AuthController(BuildConfig(), userManager.Object, Mock.Of<ITokenService>(), Mock.Of<IAppEmailSender>(), Mock.Of<IGoogleTokenValidator>(), NullLogger<AuthController>.Instance);
+        var controller = new AuthController(BuildConfig(), userManager.Object, Mock.Of<ITokenService>(), Mock.Of<IAppEmailSender>(), Mock.Of<IGoogleTokenValidator>(), Mock.Of<IMicrosoftTokenValidator>(), NullLogger<AuthController>.Instance);
 
         var result = await controller.UpdateProfile(new AuthController.UpdateProfileRequest(" new@example.com ", " newuser ", " Ada ", " Lovelace ", " Ada L. ", null, null));
 
@@ -50,7 +50,7 @@ public sealed class AuthAndSystemControllerTests
             .Setup(x => x.SendAsync(user.Email!, It.IsAny<string>(), It.IsAny<string>(), It.IsAny<CancellationToken>()))
             .ThrowsAsync(new InvalidOperationException("SMTP unavailable"));
 
-        var controller = new AuthController(BuildConfig(), userManager.Object, Mock.Of<ITokenService>(), emailSender.Object, Mock.Of<IGoogleTokenValidator>(), NullLogger<AuthController>.Instance)
+        var controller = new AuthController(BuildConfig(), userManager.Object, Mock.Of<ITokenService>(), emailSender.Object, Mock.Of<IGoogleTokenValidator>(), Mock.Of<IMicrosoftTokenValidator>(), NullLogger<AuthController>.Instance)
         {
             ControllerContext = new ControllerContext
             {
@@ -91,7 +91,7 @@ public sealed class AuthAndSystemControllerTests
             .Setup(x => x.ValidateAsync("google-token", It.IsAny<CancellationToken>()))
             .ReturnsAsync(new GoogleTokenPrincipal("google-subject", "dj@cesnimda.co.uk", true, "Dan", "Jones", "Dan Jones"));
 
-        var controller = new AuthController(BuildConfig(), userManager.Object, tokenService.Object, Mock.Of<IAppEmailSender>(), googleValidator.Object, NullLogger<AuthController>.Instance)
+        var controller = new AuthController(BuildConfig(), userManager.Object, tokenService.Object, Mock.Of<IAppEmailSender>(), googleValidator.Object, Mock.Of<IMicrosoftTokenValidator>(), NullLogger<AuthController>.Instance)
         {
             ControllerContext = new ControllerContext
             {
@@ -110,6 +110,76 @@ public sealed class AuthAndSystemControllerTests
         Assert.NotNull(user.GoogleLinkedAt);
     }
 
+    [Fact]
+    public async Task Exchange_microsoft_token_creates_new_user_when_registration_allowed()
+    {
+        var userManager = CreateUserManager();
+        userManager.Setup(x => x.Users).Returns(new TestAsyncEnumerable<ApplicationUser>(new List<ApplicationUser>()));
+        userManager.Setup(x => x.FindByEmailAsync("new.hire@example.com")).ReturnsAsync((ApplicationUser?)null);
+        ApplicationUser? created = null;
+        userManager
+            .Setup(x => x.CreateAsync(It.IsAny<ApplicationUser>()))
+            .Callback<ApplicationUser>(u => created = u)
+            .ReturnsAsync(IdentityResult.Success);
+        userManager.Setup(x => x.UpdateAsync(It.IsAny<ApplicationUser>())).ReturnsAsync(IdentityResult.Success);
+
+        var tokenService = new Mock<ITokenService>();
+        tokenService.Setup(x => x.CreateAccessTokenAsync(It.IsAny<ApplicationUser>(), It.IsAny<CancellationToken>())).ReturnsAsync("app-token");
+
+        var microsoftValidator = new Mock<IMicrosoftTokenValidator>();
+        microsoftValidator
+            .Setup(x => x.ValidateAsync("microsoft-token", It.IsAny<CancellationToken>()))
+            .ReturnsAsync(new MicrosoftTokenPrincipal("ms-subject", "new.hire@example.com", true, "New", "Hire", "New Hire"));
+
+        var config = new ConfigurationBuilder()
+            .AddInMemoryCollection(new Dictionary<string, string?> { ["Auth:AllowRegistration"] = "true" })
+            .Build();
+
+        var controller = new AuthController(config, userManager.Object, tokenService.Object, Mock.Of<IAppEmailSender>(), Mock.Of<IGoogleTokenValidator>(), microsoftValidator.Object, NullLogger<AuthController>.Instance)
+        {
+            ControllerContext = new ControllerContext
+            {
+                HttpContext = new DefaultHttpContext()
+            }
+        };
+
+        var result = await controller.ExchangeMicrosoftToken(new AuthController.MicrosoftTokenRequest("microsoft-token"), CancellationToken.None);
+
+        var ok = Assert.IsType<OkObjectResult>(result.Result);
+        var payload = Assert.IsType<AuthController.AuthSessionResult>(ok.Value);
+        Assert.True(payload.Authenticated);
+        Assert.Equal("microsoft", payload.Provider);
+        Assert.NotNull(created);
+        Assert.Equal("new.hire@example.com", created!.Email);
+        Assert.Equal("ms-subject", created.MicrosoftSubject);
+    }
+
+    [Fact]
+    public async Task Exchange_microsoft_token_rejects_unmatched_account_when_registration_disabled()
+    {
+        var userManager = CreateUserManager();
+        userManager.Setup(x => x.Users).Returns(new TestAsyncEnumerable<ApplicationUser>(new List<ApplicationUser>()));
+        userManager.Setup(x => x.FindByEmailAsync("nobody@example.com")).ReturnsAsync((ApplicationUser?)null);
+
+        var microsoftValidator = new Mock<IMicrosoftTokenValidator>();
+        microsoftValidator
+            .Setup(x => x.ValidateAsync("microsoft-token", It.IsAny<CancellationToken>()))
+            .ReturnsAsync(new MicrosoftTokenPrincipal("ms-subject", "nobody@example.com", true, null, null, null));
+
+        var controller = new AuthController(BuildConfig(), userManager.Object, Mock.Of<ITokenService>(), Mock.Of<IAppEmailSender>(), Mock.Of<IGoogleTokenValidator>(), microsoftValidator.Object, NullLogger<AuthController>.Instance)
+        {
+            ControllerContext = new ControllerContext
+            {
+                HttpContext = new DefaultHttpContext()
+            }
+        };
+
+        var result = await controller.ExchangeMicrosoftToken(new AuthController.MicrosoftTokenRequest("microsoft-token"), CancellationToken.None);
+
+        Assert.IsType<UnauthorizedObjectResult>(result.Result);
+        userManager.Verify(x => x.CreateAsync(It.IsAny<ApplicationUser>()), Times.Never);
+    }
+
     [Fact]
     public void Me_result_includes_google_link_details_for_local_users()
     {

JobTrackerApi.Tests/ClientErrorsControllerTests.cs

@@ -51,7 +51,7 @@ public sealed class ClientErrorsControllerTests
         var userManager = TestHostFactory.CreateUserManager();
         userManager.Setup(x => x.GetUserAsync(It.IsAny<ClaimsPrincipal>())).ReturnsAsync(user);
 
-        var controller = new AuthController(BuildConfig(), userManager.Object, Mock.Of<ITokenService>(), Mock.Of<IAppEmailSender>(), Mock.Of<IGoogleTokenValidator>(), Mock.Of<ILogger<AuthController>>())
+        var controller = new AuthController(BuildConfig(), userManager.Object, Mock.Of<ITokenService>(), Mock.Of<IAppEmailSender>(), Mock.Of<IGoogleTokenValidator>(), Mock.Of<IMicrosoftTokenValidator>(), Mock.Of<ILogger<AuthController>>())
         {
             ControllerContext = new ControllerContext
             {

JobTrackerApi.Tests/MicrosoftTokenValidatorTests.cs

@@ -0,0 +1,77 @@
+using System.IdentityModel.Tokens.Jwt;
+using System.Security.Claims;
+using System.Text;
+using JobTrackerApi.Services;
+using Microsoft.Extensions.Configuration;
+using Microsoft.IdentityModel.Protocols;
+using Microsoft.IdentityModel.Protocols.OpenIdConnect;
+using Microsoft.IdentityModel.Tokens;
+using Moq;
+using Xunit;
+
+namespace JobTrackerApi.Tests;
+
+public sealed class MicrosoftTokenValidatorTests
+{
+    private static (IConfiguration Config, Mock<IConfigurationManager<OpenIdConnectConfiguration>> ConfigManager, SymmetricSecurityKey Key) BuildHarness()
+    {
+        var config = new ConfigurationBuilder()
+            .AddInMemoryCollection(new Dictionary<string, string?> { ["Auth:MicrosoftClientId"] = "client-123" })
+            .Build();
+
+        var signingKey = new SymmetricSecurityKey(Encoding.UTF8.GetBytes("super-secret-signing-key-super-secret"));
+        var oidc = new OpenIdConnectConfiguration();
+        oidc.SigningKeys.Add(signingKey);
+
+        var configManager = new Mock<IConfigurationManager<OpenIdConnectConfiguration>>();
+        configManager.Setup(x => x.GetConfigurationAsync(It.IsAny<CancellationToken>())).ReturnsAsync(oidc);
+
+        return (config, configManager, signingKey);
+    }
+
+    [Fact]
+    public async Task ValidateAsync_accepts_tenant_scoped_issuer_and_maps_oid_to_subject()
+    {
+        var (config, configManager, signingKey) = BuildHarness();
+
+        var token = new JwtSecurityTokenHandler().WriteToken(new JwtSecurityToken(
+            issuer: "https://login.microsoftonline.com/9f2c1e3a-tenant/v2.0",
+            audience: "client-123",
+            claims: new[]
+            {
+                new Claim("oid", "ms-subject-1"),
+                new Claim("email", "demo@example.com"),
+                new Claim("given_name", "Demo"),
+                new Claim("family_name", "User"),
+                new Claim("name", "Demo User"),
+            },
+            expires: DateTime.UtcNow.AddMinutes(10),
+            signingCredentials: new SigningCredentials(signingKey, SecurityAlgorithms.HmacSha256)));
+
+        var validator = new MicrosoftTokenValidator(config, configManager.Object);
+        var result = await validator.ValidateAsync(token);
+
+        Assert.Equal("ms-subject-1", result.Subject);
+        Assert.Equal("demo@example.com", result.Email);
+        Assert.True(result.EmailVerified);
+        Assert.Equal("Demo", result.GivenName);
+        Assert.Equal("User", result.FamilyName);
+    }
+
+    [Fact]
+    public async Task ValidateAsync_rejects_non_microsoft_issuer()
+    {
+        var (config, configManager, signingKey) = BuildHarness();
+
+        var token = new JwtSecurityTokenHandler().WriteToken(new JwtSecurityToken(
+            issuer: "https://evil.example.com/v2.0",
+            audience: "client-123",
+            claims: new[] { new Claim("oid", "ms-subject-1") },
+            expires: DateTime.UtcNow.AddMinutes(10),
+            signingCredentials: new SigningCredentials(signingKey, SecurityAlgorithms.HmacSha256)));
+
+        var validator = new MicrosoftTokenValidator(config, configManager.Object);
+
+        await Assert.ThrowsAsync<InvalidOperationException>(() => validator.ValidateAsync(token));
+    }
+}