perf(qbittorrent): rid incremental sync + shared cache + backoff (stop hanging qBittorrent)

The app was saturating qBittorrent's single-threaded web server and causing
its own Web UI (and the reverse proxy) to hang/504: each of the 3 qBittorrent
widgets fetched /sync/maindata independently, every call was a FULL snapshot
(no rid), and polling was aggressive (5s for speed). For large torrent lists
each snapshot is heavy, so the server queued and Traefik timed out.

QbittorrentClient.maindata now:
- Uses the incremental rid protocol: the first call is a full_update;
  subsequent calls send the last rid and get a small diff that is merged into
  a cached snapshot (full_update replaces; partial_update merges server_state,
  torrents {added/None-removed/..._removed}, categories, tags, trackers).
  Payloads shrink dramatically for large libraries.
- Serves a short-TTL (3s) cached snapshot under a lock, so concurrent widget
  polls collapse onto a single HTTP fetch instead of N.
- Backs off exponentially (capped 30s) on repeated failure, serving the last
  good snapshot when available, so a struggling qBittorrent isn't hammered
  further. Returns a shallow race-safe copy of the snapshot per call.

Also slow the speed widget poll from 5s -> 15s (backend widget-kind +
frontend registry) for ~3x fewer calls.

Tests: rid full+partial merge, cache collapses within-TTL calls, backoff
skips the network after failure and serves stale. 393/393 backend + 180/180
frontend tests pass; ruff + tsc + ESLint clean.

backend/src/media_library_viewer_api/integrations/.pi-map.index.md

@@ -2,7 +2,7 @@
 dir: backend/src/media_library_viewer_api/integrations
 
 ## role
-Provides pluggable external service integrations (Alertmanager, Jellyfin, Nextcloud, Prometheus, qBittorrent, Authentik, Backups, SSH) with standardized config schemas, widgets, connection testing, and a central registry for discovery.
+Provides pluggable external service integrations (e.g., Alertmanager, Jellyfin, Prometheus, qBittorrent) with standardized configuration, connection testing, and widget definitions for a monitoring dashboard.
 ## parent
 index: backend/src/media_library_viewer_api/.pi-map.index.md
 map: backend/src/media_library_viewer_api/.pi-map.md