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/AccountDeletionTests.cs

@@ -111,6 +111,17 @@ public sealed class AccountDeletionTests
             });
             var otherOperation = Operation(other.Id, OperationStatuses.Queued);
             fixture.Db.UserOperations.Add(otherOperation);
+            fixture.Db.AiUsageRecords.AddRange(
+                new AiUsageRecord
+                {
+                    OwnerUserId = owner.Id, SourceType = "operation", SourceId = "owner-usage",
+                    TaskType = "synthetic", EstimatedTokenCount = 100, CreatedAtUtc = DateTimeOffset.UtcNow,
+                },
+                new AiUsageRecord
+                {
+                    OwnerUserId = other.Id, SourceType = "operation", SourceId = "other-usage",
+                    TaskType = "synthetic", EstimatedTokenCount = 100, CreatedAtUtc = DateTimeOffset.UtcNow,
+                });
             await fixture.Db.SaveChangesAsync();
             var generatedRoot = Path.Combine(fixture.Paths.GetOwnerCvExportsRoot(owner.Id), "20260815");
             Directory.CreateDirectory(generatedRoot);
@@ -132,6 +143,8 @@ public sealed class AccountDeletionTests
             Assert.False(await fixture.Db.JobApplications.IgnoreQueryFilters().AnyAsync(item => item.OwnerUserId == owner.Id));
             Assert.True(await fixture.Db.JobApplications.IgnoreQueryFilters().AnyAsync(item => item.OwnerUserId == other.Id));
             Assert.True(await fixture.Db.UserOperations.IgnoreQueryFilters().AnyAsync(item => item.Id == otherOperation.Id));
+            Assert.False(await fixture.Db.AiUsageRecords.IgnoreQueryFilters().AnyAsync(item => item.OwnerUserId == owner.Id));
+            Assert.True(await fixture.Db.AiUsageRecords.IgnoreQueryFilters().AnyAsync(item => item.OwnerUserId == other.Id));
             Assert.False(File.Exists(attachmentPath));
             Assert.False(File.Exists(artifactPath));
             Assert.False(File.Exists(generatedPath));