add database backup and restore admin page

- SQLCipher-encrypted backup creation via VACUUM INTO
- Backup history with auto-pruning (keeps last 5)
- Pre-restore automatic backup for safety
- Restore from history or uploaded file
- Stats display with table breakdown
- Download hook for client-side file download
- SECRET_KEY_DB config for encryption at rest

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

.gitignore

@@ -72,6 +72,7 @@ package-lock.json
 
 # Environment variables (API tokens, secrets)
 .env
+.envrc
 
 # API reference specs (development only)
 /docs/api-specs/