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/Services/AccountDataExportService.cs

@@ -174,6 +174,7 @@ public sealed class AccountDataExportService(
                 var interviewNotes = await db.InterviewPrepNotes.IgnoreQueryFilters().AsNoTracking().Where(item => item.OwnerUserId == ownerUserId).OrderBy(item => item.Id).ToListAsync(cancellationToken);
                 var aiNotes = await db.AiWorkspaceNotes.IgnoreQueryFilters().AsNoTracking().Where(item => item.OwnerUserId == ownerUserId).OrderBy(item => item.Id).ToListAsync(cancellationToken);
                 var aiInteractions = await db.AiInteractions.IgnoreQueryFilters().AsNoTracking().Where(item => item.OwnerUserId == ownerUserId).OrderBy(item => item.Id).ToListAsync(cancellationToken);
+                var aiUsage = await db.AiUsageRecords.IgnoreQueryFilters().AsNoTracking().Where(item => item.OwnerUserId == ownerUserId).OrderBy(item => item.Id).ToListAsync(cancellationToken);
                 var checklist = await db.ApplicationChecklistItems.IgnoreQueryFilters().AsNoTracking().Where(item => item.OwnerUserId == ownerUserId).OrderBy(item => item.Id).ToListAsync(cancellationToken);
                 var coverLetters = await db.CoverLetterVersions.IgnoreQueryFilters().AsNoTracking().Where(item => item.OwnerUserId == ownerUserId).OrderBy(item => item.Id).ToListAsync(cancellationToken);
                 var interviewItems = await db.InterviewPrepItems.IgnoreQueryFilters().AsNoTracking().Where(item => item.OwnerUserId == ownerUserId).OrderBy(item => item.Id).ToListAsync(cancellationToken);
@@ -187,12 +188,13 @@ public sealed class AccountDataExportService(
                     InterviewPrepNotes = interviewNotes,
                     AiWorkspaceNotes = aiNotes,
                     AiInteractions = aiInteractions,
+                    AiUsage = aiUsage,
                     ChecklistItems = checklist,
                     CoverLetterVersions = coverLetters,
                     InterviewPrepItems = interviewItems,
                     EmailDrafts = emailDrafts,
                     EmailSendAttempts = emailAttempts,
-                }, tailoredDrafts.Count + interviewNotes.Count + aiNotes.Count + aiInteractions.Count + checklist.Count + coverLetters.Count + interviewItems.Count + emailDrafts.Count + emailAttempts.Count);
+                }, tailoredDrafts.Count + interviewNotes.Count + aiNotes.Count + aiInteractions.Count + aiUsage.Count + checklist.Count + coverLetters.Count + interviewItems.Count + emailDrafts.Count + emailAttempts.Count);
 
                 var operations = await db.UserOperations.IgnoreQueryFilters().AsNoTracking().Where(item => item.OwnerUserId == ownerUserId).OrderBy(item => item.CreatedAtUtc)
                     .Select(item => new