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/StrategySnapshotOperationTests.cs

@@ -60,6 +60,12 @@ public sealed class StrategySnapshotOperationTests
         Assert.Equal("ollama", operation.Provider);
         Assert.Equal("qwen-test", operation.Model);
         Assert.Equal(1, fixture.GenerationCalls);
+        var usage = Assert.Single(await db.AiUsageRecords.ToListAsync());
+        Assert.Equal("strategy.snapshot", usage.TaskType);
+        Assert.True(usage.InputCharacterCount > 0);
+        Assert.Equal(Fixture.ValidResponse.Length, usage.OutputCharacterCount);
+        Assert.Equal((usage.InputCharacterCount + usage.OutputCharacterCount + 3) / 4, usage.EstimatedTokenCount);
+        Assert.True(usage.EstimatedTokenCount < AiUsageMeter.ReservationFor(StrategySnapshotService.TaskType).EstimatedTokens);
     }
 
     [Fact]
@@ -103,7 +109,7 @@ public sealed class StrategySnapshotOperationTests
 
     private sealed class Fixture : IAsyncDisposable
     {
-        private const string ValidResponse = """{"strategicSummary":"Lead with delivery evidence.","cvBulletIdeas":["Built APIs"],"proofPointsToLeadWith":["Led delivery"],"coverLetterAngles":["Platform relevance"]}""";
+        internal const string ValidResponse = """{"strategicSummary":"Lead with delivery evidence.","cvBulletIdeas":["Built APIs"],"proofPointsToLeadWith":["Led delivery"],"coverLetterAngles":["Platform relevance"]}""";
         private readonly SqliteConnection _connection;
         private readonly Mock<ISummarizerService> _summarizer;
         public ServiceProvider Provider { get; }
@@ -146,6 +152,7 @@ public sealed class StrategySnapshotOperationTests
             services.AddSingleton<AiPrivacyPolicy>();
             services.AddSingleton<AiOperationExecutionScope>();
             services.AddScoped<UserOperationStore>();
+            services.AddScoped<AiUsageMeter>();
             services.AddScoped<AiOperationAdmission>();
             services.AddScoped<StrategySnapshotService>();
             services.AddSingleton<IAiOperationHandler, StrategySnapshotOperationHandler>();