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/AiOperationQueueTests.cs

@@ -34,6 +34,10 @@ public sealed class AiOperationQueueTests
         Assert.Equal("local_only", first.Operation.PrivacyPolicy);
         Assert.Equal("ai_queue_full", full.Code);
         Assert.Equal(15, full.RetryAfterSeconds);
+        var usage = Assert.Single(await scope.ServiceProvider.GetRequiredService<JobTrackerContext>()
+            .AiUsageRecords.IgnoreQueryFilters().AsNoTracking().ToListAsync());
+        Assert.Equal(first.Operation.Id.ToString("D"), usage.SourceId);
+        Assert.Equal(4_000, usage.EstimatedTokenCount);
     }
 
     [Fact]
@@ -137,7 +141,8 @@ public sealed class AiOperationQueueTests
             if (GenerationFailure is not null) throw GenerationFailure;
             OwnerUserId = services.GetRequiredService<ICurrentUserService>().UserId;
             PrivacyPolicy = context.EffectivePrivacyPolicy;
-            return Task.FromResult(new AiOperationExecutionResult("synthetic-result", "ollama", "qwen-test", "local_primary"));
+            return Task.FromResult(new AiOperationExecutionResult(
+                "synthetic-result", "ollama", "qwen-test", "local_primary", 120, 40));
         }
     }
 
@@ -178,6 +183,7 @@ public sealed class AiOperationQueueTests
             services.AddScoped<UserOperationStore>();
             services.AddSingleton<AiPrivacyPolicy>();
             services.AddSingleton<AiOperationExecutionScope>();
+            services.AddScoped<AiUsageMeter>();
             services.AddScoped<AiOperationAdmission>();
             services.AddSingleton<IAiOperationHandler>(handler);
             services.AddSingleton<AiOperationWorker>();