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/CvProcessingOperationTests.cs

@@ -51,6 +51,9 @@ public sealed class CvProcessingOperationTests
             Assert.Equal(CvProcessingQueue.TaskType, operation.TaskType);
             Assert.Equal(CvProcessingQueue.SubjectType, operation.SubjectType);
             Assert.DoesNotContain("Ada", operation.SubjectId ?? string.Empty, StringComparison.OrdinalIgnoreCase);
+            var usage = Assert.Single(await db.AiUsageRecords.IgnoreQueryFilters().ToListAsync());
+            Assert.Equal(operation.Id.ToString("D"), usage.SourceId);
+            Assert.Equal(AiUsageMeter.ReservationFor(CvProcessingQueue.TaskType).EstimatedTokens, usage.EstimatedTokenCount);
         }
 
         Assert.True(await fixture.Provider.GetRequiredService<AiOperationWorker>().RunOnceAsync(default));
@@ -68,6 +71,8 @@ public sealed class CvProcessingOperationTests
             Assert.Null(user.ProfileCvStructureJson);
             Assert.Null(user.CurrentCvExtractionRunId);
             Assert.Equal("operation_succeeded", (await db.UserNotifications.IgnoreQueryFilters().SingleAsync()).Kind);
+            Assert.Equal(AiUsageMeter.ReservationFor(CvProcessingQueue.TaskType).EstimatedTokens,
+                (await db.AiUsageRecords.IgnoreQueryFilters().SingleAsync()).EstimatedTokenCount);
         }
     }
 
@@ -263,6 +268,7 @@ public sealed class CvProcessingOperationTests
             services.AddSingleton<AiPrivacyPolicy>();
             services.AddSingleton<AiOperationExecutionScope>();
             services.AddScoped<UserOperationStore>();
+            services.AddScoped<AiUsageMeter>();
             services.AddScoped<AiOperationAdmission>();
             services.AddScoped<ICvProcessingQueue, CvProcessingQueue>();
             services.AddTransient<ProfileCvController>();