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/settings.rs

@@ -88,7 +88,7 @@ fn default_profile() -> String {
     "default".to_string()
 }
 
-/// WebDAV v2 同步设置
+/// WebDAV 同步设置
 #[derive(Debug, Clone, Serialize, Deserialize)]
 #[serde(rename_all = "camelCase")]
 pub struct WebDavSyncSettings {