fix: Implement tiered backup retention classification (refs #6)

Replace per-schedule count-based retention with unified tiered classification.
Backups are now classified by timestamp into categories (hourly/daily/weekly/monthly)
and are only deleted when they exceed ALL applicable category quotas.

Changes:
- Add backup-classification.service.ts for timestamp-based classification
- Rewrite backup-retention.service.ts with tiered logic
- Add categories and expires_at columns to backup_history
- Add Expires column to desktop and mobile backup UI
- Add unit tests for classification logic (22 tests)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/src/features/backup/CLAUDE.md

@@ -0,0 +1,30 @@
+# backup/
+
+Complete backup and restore system with tiered retention.
+
+## Files
+
+| File | What | When to read |
+| ---- | ---- | ------------ |
+| `README.md` | Feature documentation | Understanding backup architecture |
+
+## Subdirectories
+
+| Directory | What | When to read |
+| --------- | ---- | ------------ |
+| `api/` | HTTP endpoints, validation | API changes |
+| `domain/` | Business logic, services | Core backup/retention logic |
+| `data/` | Repository, database queries | Database operations |
+| `jobs/` | Scheduled job handlers | Cron job modifications |
+| `migrations/` | Database schema | Schema changes |
+| `tests/` | Unit and integration tests | Adding or modifying tests |
+
+## Key Files
+
+| File | What | When to read |
+| ---- | ---- | ------------ |
+| `domain/backup.types.ts` | Types, constants, TIERED_RETENTION | Type definitions |
+| `domain/backup.service.ts` | Core backup operations | Creating/managing backups |
+| `domain/backup-classification.service.ts` | Tiered retention classification | Category/expiration logic |
+| `domain/backup-retention.service.ts` | Retention enforcement | Deletion logic |
+| `data/backup.repository.ts` | Database queries | Data access patterns |