Add automated backup system with SQLite VACUUM

- Implement backup package with SQLite VACUUM INTO for safe backups
- Add scheduler with configurable interval (default: 24h)
- Add retention policy with automatic cleanup (default: 7 days)
- Add optional gzip compression (~10x size reduction)
- Add comprehensive tests for backup creation and cleanup
- Integrate backup scheduler in main.go with graceful shutdown
- Add backup configuration variables (BACKUP_ENABLED, BACKUP_INTERVAL, BACKUP_RETENTION_DAYS, BACKUP_PATH, BACKUP_COMPRESS)
- Backups run automatically in background goroutine
- Coverage: backup package 52.7%

.env.example

@@ -29,3 +29,10 @@ REGISTRATION_MODE=open
 # Logging Configuration
 LOG_LEVEL=info
 ENVIRONMENT=production
+
+# Backup Configuration
+BACKUP_ENABLED=false
+BACKUP_INTERVAL=24h
+BACKUP_RETENTION_DAYS=7
+BACKUP_PATH=./data/backups
+BACKUP_COMPRESS=true