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/Program.cs

@@ -52,6 +52,7 @@ builder.Services.AddSingleton<AccountDeletionTombstoneStore>();
 builder.Services.AddScoped<IAiSidecarCachePurger, AiSidecarCachePurger>();
 builder.Services.AddScoped<AccountDeletionService>();
 builder.Services.AddScoped<AiOperationAdmission>();
+builder.Services.AddScoped<AiUsageMeter>();
 builder.Services.AddScoped<StrategySnapshotService>();
 builder.Services.AddSingleton<IAiOperationHandler, StrategySnapshotOperationHandler>();
 builder.Services.AddSingleton<IAiOperationHandler, CvProcessingOperationHandler>();