feat(ai): centralize durable usage

Add a content-free usage ledger with legacy backfill. Reserve Workspace and durable Strategy/CV work before execution so deleted history or duplicate admission cannot reset limits.

JobTrackerApi/Controllers/AiWorkspaceController.cs

@@ -19,13 +19,15 @@ public sealed class AiWorkspaceController : ControllerBase
     private readonly IAiWorkspaceService _workspace;
     private readonly IConfiguration _config;
     private readonly JobTrackerApi.Data.JobTrackerContext? _db;
+    private readonly AiUsageMeter? _usage;
 
-    public AiWorkspaceController(UserManager<ApplicationUser> users, IAiWorkspaceService workspace, IConfiguration config, JobTrackerApi.Data.JobTrackerContext? db = null)
+    public AiWorkspaceController(UserManager<ApplicationUser> users, IAiWorkspaceService workspace, IConfiguration config, JobTrackerApi.Data.JobTrackerContext? db = null, AiUsageMeter? usage = null)
     {
         _users = users;
         _workspace = workspace;
         _config = config;
         _db = db;
+        _usage = usage;
     }
 
     public sealed record GenerateRequest(string Module, string? Mode, string? ExtraContext);
@@ -42,38 +44,29 @@ public sealed class AiWorkspaceController : ControllerBase
         if (user is null) return Unauthorized();
         if (string.IsNullOrWhiteSpace(request?.Module)) return BadRequest("Choose an AI module.");
 
+        AiUsageReservation? reservation = null;
         if (_db is not null)
         {
             var roles = await _users.GetRolesAsync(user);
             var entitlements = AccountPlans.ForRoles(roles);
-            var monthStart = new DateTimeOffset(DateTime.UtcNow.Year, DateTime.UtcNow.Month, 1, 0, 0, 0, TimeSpan.Zero);
-            var interactions = _db.AiInteractions.Where(x => x.OwnerUserId == user.Id);
-            int usedCalls;
-            long usedTokens;
-            if (_db.Database.IsSqlite())
+            var usage = _usage ?? new AiUsageMeter(_db, TimeProvider.System);
+            var estimate = AiUsageMeter.ReservationFor($"workspace.{request.Module.Trim().ToLowerInvariant()}");
+            try
             {
-                var used = (await interactions
-                    .Select(x => new { x.CreatedAtUtc, x.EstimatedTokenCount })
-                    .ToListAsync(ct))
-                    .Where(x => x.CreatedAtUtc >= monthStart)
-                    .ToList();
-                usedCalls = used.Count;
-                usedTokens = used.Sum(x => (long)x.EstimatedTokenCount);
+                reservation = await usage.ReserveAsync(
+                    user.Id,
+                    entitlements,
+                    "workspace",
+                    Guid.NewGuid().ToString("D"),
+                    $"workspace.{request.Module.Trim().ToLowerInvariant()}",
+                    estimate.InputCharacters,
+                    estimate.EstimatedTokens,
+                    ct);
             }
-            else
+            catch (AiUsageLimitException ex)
             {
-                var used = await interactions
-                    .Where(x => x.CreatedAtUtc >= monthStart)
-                    .GroupBy(_ => 1)
-                    .Select(g => new { Calls = g.Count(), Tokens = g.Sum(x => (long)x.EstimatedTokenCount) })
-                    .FirstOrDefaultAsync(ct);
-                usedCalls = used?.Calls ?? 0;
-                usedTokens = used?.Tokens ?? 0;
+                return StatusCode(StatusCodes.Status429TooManyRequests, ex.Message);
             }
-            if (usedCalls >= entitlements.MonthlyAiCalls)
-                return StatusCode(StatusCodes.Status429TooManyRequests, $"Monthly AI limit reached ({entitlements.MonthlyAiCalls} generations). Upgrade your plan or try again next month.");
-            if (usedTokens >= entitlements.MonthlyAiTokens)
-                return StatusCode(StatusCodes.Status429TooManyRequests, $"Monthly AI cost limit reached ({entitlements.MonthlyAiTokens:N0} estimated tokens). Upgrade your plan or try again next month.");
         }
 
         try
@@ -81,10 +74,19 @@ public sealed class AiWorkspaceController : ControllerBase
             var interaction = await _workspace.GenerateAsync(
                 user.Id, jobId, user.ProfileCvText ?? string.Empty, ResolveName(user),
                 new AiGenerateRequest(request.Module, request.Mode, request.ExtraContext), ResolveProvider(), ct);
-            return interaction is null ? NotFound() : Ok(ToDto(interaction));
+            if (interaction is null)
+            {
+                if (reservation is not null) await (_usage ?? new AiUsageMeter(_db!, TimeProvider.System)).ReleaseAsync(reservation.Record.Id, ct);
+                return NotFound();
+            }
+            if (reservation is not null)
+                await (_usage ?? new AiUsageMeter(_db!, TimeProvider.System)).FinalizeAsync(
+                    reservation.Record.Id, interaction.InputCharacterCount, interaction.OutputCharacterCount, ct);
+            return Ok(ToDto(interaction));
         }
         catch (ArgumentException ex)
         {
+            if (reservation is not null) await (_usage ?? new AiUsageMeter(_db!, TimeProvider.System)).ReleaseAsync(reservation.Record.Id, ct);
             return BadRequest(ex.Message);
         }
         catch (AiUnavailableException ex)