fix(qbittorrent): poll only what changed via /sync/maindata

[v3DJG6GL]

Issue

Every 3 seconds the torrent list downloaded every torrent from /torrents/info and parsed all of them again. With thousands of torrents that's slow and uses a lot of data. On top of that, pollEvery ticks on a fixed timer, so when a fetch took longer than the interval, the next tick restarted it and the spinner never went away.

Changes

• The list now uses /sync/maindata, the same endpoint qBittorrent's web UI uses. The first call returns everything. After that, it returns only changed fields, new torrents and removed hashes.
• QbitSyncStore keeps the merged list per instance. It isn't autoDispose, so reopening the screen continues from the last rid instead of fetching the full list again.
• Syncs run one at a time, so an older reply can't land after a newer one and overwrite it.
• If a reply fails to merge, rid goes back to 0 and the next poll gets the full list.
• Every qBittorrent poll (list, transfer info, properties, files, trackers, peers) now uses ref.polled instead of pollEvery.
• The list interval is set per instance with the existing "Polling Interval" field on the instance form, which now shows for qBittorrent. Detail screens stay at 10s.
• Behavior change: the default list interval is now 5s (the instance default), up from a fixed 3s. Anything under 1s, e.g. from an imported profile, is raised to 1s.

Size

	added	removed
code	94	36
comments	42	8
tests	302	0
docs	3	2

Testing

• flutter analyze is clean for service_qbittorrent and app. All 73 tests in service_qbittorrent pass, as well as other qBittorrent and polling tests.
• New tests: full update, partial delta (every model field survives a merge), added and removed torrents, a full update dropping stale torrents, failed merge resets rid, overlapping syncs wait and use the newer rid, a failed sync doesn't block the next one, reopening resumes the delta, interval comes from the instance with a 1s floor, and the form shows the field for qBittorrent.

LLM Disclosure

Developed with assistance from Claude Code (Opus 5.5). The LLM helped me identify the related files, code parts and proposed possible code modifications and fixes.
I've reviewed, cleaned, refactored and hardened the code the best I could, but my Dart capabilities are not that good.

[Bhavyashah94]

Tested this PR against a live qBittorrent instance (v5.2.4) with ~75 torrents, as well as running on an Android emulator (Pixel 7 / API 35). Leaving a review comment with test findings for other testers and maintainers.

Summary of Testing

1. Delta Sync & Store Logic (QbitSyncStore):

   • Verified initial sync behavior (rid: 0 receives full_update: true and parses all torrents).
   • Subsequent polling ticks query /sync/maindata?rid=$rid and only receive changed fields (e.g. download/upload speeds, progress, eta), cutting down bandwidth significantly compared to polling /torrents/info.
   • Verified that partial field patches correctly preserve unchanged model properties (added_on, category, save_path, magnet_uri, etc.) across JSON merge roundtrips.
   • Verified sequential request queuing via _tail Future chaining to prevent overlapping calls (e.g. manual refresh vs timer tick) from racing or winding back rid.
   • Verified that if a malformed patch fails to apply, rid resets to 0 so the next poll recovers with a clean full fetch.

2. Polling Interval Configuration:

   • Verified that the InstanceFormScreen now renders the Polling section and Polling Interval (seconds) field when configuring a qBittorrent instance (defaulting to 5s).
   • Verified the interval logic enforces a 1s minimum (max(1, instance.pollingIntervalSeconds)).

3. Android Client UI (Emulator):

   • Dashboard: "Active downloads" card updates live download speeds smoothly without spinner thrashing.
   • Activity Tab: Real-time speed updates and download progress render reactively.
   • Torrent List: Torrents render with stats, status badges, and search without full-list redraw flicker on every tick.
   • Torrent Details: Detail sheets (Overview, Files, Trackers, Peers) open reliably and remain consistent with the synced state.

4. Automated Tests:

   • All 73 tests in services/service_qbittorrent pass (including the new qbit_sync_store_test.dart suite).
   • Form widget test (gluetun_form_polling_test.dart) passes.
   • dart analyze / flutter analyze reports 0 issues.

Everything looks solid and operates cleanly on live data. Leaving review open without approval so other testers can also test and verify on their setups.