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.

frontend/src/integrations/registry.ts

@@ -119,7 +119,12 @@ export const SERVICE_REGISTRY: Record<string, ServiceBinding> = {
 				name: "Chart",
 				description: "Multi-series line chart from a PromQL range query.",
 				refreshIntervalMs: 60_000,
-				defaultConfig: { promql: "", window: "1h", unit: "none", scale: "auto" },
+				defaultConfig: {
+					promql: "",
+					window: "1h",
+					unit: "none",
+					scale: "auto",
+				},
 				configSchema: {
 					type: "object",
 					properties: {
@@ -213,7 +218,7 @@ export const SERVICE_REGISTRY: Record<string, ServiceBinding> = {
 				kind: "speed",
 				name: "Speed chart",
 				description: "Live download/upload speed over a short window.",
-				refreshIntervalMs: 5_000,
+				refreshIntervalMs: 15_000,
 				defaultConfig: { unit: "bytes_per_sec", scale: "auto" },
 				configSchema: {
 					type: "object",