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.Tests/SqliteDateTimeOffsetCompatibilityTests.cs

@@ -100,6 +100,10 @@ public sealed class SqliteDateTimeOffsetCompatibilityTests
             Interaction("user-1", job.Id, now),
             Interaction("user-1", job.Id, now.AddMonths(-2)),
             Interaction("user-2", job.Id, now));
+        db.AiUsageRecords.AddRange(
+            Usage("user-1", "current", now),
+            Usage("user-1", "old", now.AddMonths(-2)),
+            Usage("user-2", "other", now));
         db.CvExtractionRuns.AddRange(
             Run("user-1", now.AddDays(-1), "old"),
             Run("user-1", now, "new"),
@@ -116,6 +120,7 @@ public sealed class SqliteDateTimeOffsetCompatibilityTests
         var generate = await new AiWorkspaceController(users.Object, workspaceService.Object, new ConfigurationBuilder().Build(), db)
             .Generate(job.Id, new AiWorkspaceController.GenerateRequest("job-analysis", null, null), default);
         Assert.IsType<NotFoundResult>(generate.Result);
+        Assert.Equal(2, await db.AiUsageRecords.CountAsync());
 
         var newestArtifactPath = Path.Combine(fixture.Paths.CvArtifactsRoot, "new.txt");
         await File.WriteAllTextAsync(newestArtifactPath, "synthetic CV");
@@ -146,6 +151,19 @@ public sealed class SqliteDateTimeOffsetCompatibilityTests
         UpdatedAtUtc = updated,
     };
 
+    private static AiUsageRecord Usage(string owner, string sourceId, DateTimeOffset created) => new()
+    {
+        OwnerUserId = owner,
+        SourceType = "test",
+        SourceId = sourceId,
+        TaskType = "workspace.test",
+        CallCount = 1,
+        InputCharacterCount = 10,
+        OutputCharacterCount = 5,
+        EstimatedTokenCount = 4,
+        CreatedAtUtc = created,
+    };
+
     private static AiInteraction Interaction(string owner, int jobId, DateTimeOffset created) => new()
     {
         OwnerUserId = owner,