feat: add usage daily rollups, incremental auto-vacuum, and sync-aware backup

- Add usage_daily_rollups table (schema v6) to aggregate proxy request
  logs into daily summaries, reducing query overhead for statistics
- Add rollup_and_prune DAO that aggregates old detail logs (>N days)
  into rollup rows and deletes the originals
- Update all usage stats queries to UNION detail logs with rollup data
- Introduce incremental auto-vacuum for SQLite, with startup and
  periodic cleanup of old stream_check_logs and request log rollups
- Split backup export/import into full vs sync variants: WebDAV sync
  now skips local-only table data (proxy_request_logs,
  stream_check_logs, provider_health, proxy_live_backup,
  usage_daily_rollups) while preserving them on import
- Add enable_logging guard to skip request log writes when disabled
- Apply cargo fmt formatting fixes across multiple modules

src-tauri/src/database/dao/mod.rs

@@ -11,6 +11,7 @@ pub mod settings;
 pub mod skills;
 pub mod stream_check;
 pub mod universal_providers;
+pub mod usage_rollup;
 
 // 所有 DAO 方法都通过 Database impl 提供，无需单独导出
 // 导出 FailoverQueueItem 供外部使用