fix(qbittorrent): serve stale app info/preferences when refresh fails

[s0up4200]

When a qBittorrent WebUI is saturated, the per-torrent-stream GetAppInfo/GetAppPreferences refreshes time out and currently fail the whole stream tick (the Failed to retrieve qBittorrent app info/preferences for torrent stream log noise). This serves the last-known cached value on refresh error instead, surfacing the error only when nothing is cached — mirroring the existing active_tasks serve-stale pattern.

Related to #2052 (broader graceful-degradation work); this is the small standalone piece.

Summary by CodeRabbit

Release Notes

• Bug Fixes

  • Enhanced error handling to gracefully serve previously cached data when refresh operations fail, improving app resilience and availability instead of returning errors immediately.

• Tests

  • Added test coverage for cache fallback scenarios during refresh failures.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro

Run ID: df23e4ca-bb57-4d00-ad3f-7c5ca003b041

📥 Commits

Reviewing files that changed from the base of the PR and between c7e08b9 and 5d9bc4f.

📒 Files selected for processing (4)

• internal/qbittorrent/app_info.go
• internal/qbittorrent/app_info_test.go
• internal/qbittorrent/preferences.go
• internal/qbittorrent/preferences_test.go

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Walkthrough

GetAppInfo and GetAppPreferences are both refactored to clone and evaluate their cached state under the read lock, release the lock, and—when a cache refresh fails—return the stale cached value with a debug log instead of propagating the error. If no cached value exists, the refresh error is still returned. Two new test files cover both branches for each method.

Changes

Stale cache fallback for AppInfo and AppPreferences

Layer / File(s)	Summary
GetAppInfo stale-cache fallback internal/qbittorrent/app_info.go	Clones cached AppInfo and computes freshness under RLock; on refresh failure returns stale cache with debug log (including instanceID), or returns the error when no cache exists.
GetAppInfo tests internal/qbittorrent/app_info_test.go	Adds TestGetAppInfoServesStaleCacheWhenRefreshFails and TestGetAppInfoReturnsErrorWhenNoCacheAndRefreshFails, covering the stale-fallback and no-cache-error paths via a pre-canceled context.
GetAppPreferences stale-cache fallback internal/qbittorrent/preferences.go	Same structural refactor as GetAppInfo: clones and checks freshness under preferencesMu.RLock, falls back to stale preferences with zerolog/log debug log on refresh failure, or returns the error with no cache.
GetAppPreferences tests internal/qbittorrent/preferences_test.go	New test file with TestGetAppPreferencesServesStaleCacheWhenRefreshFails and TestGetAppPreferencesReturnsErrorWhenNoCacheAndRefreshFails, mirroring the same two scenarios.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Possibly related issues

• Gracefully degrade when qBittorrent WebUI is slow/saturated (keep cached torrents + staleness indicator) #2052: The changes directly implement the serve-stale cache fix for GetAppInfo and GetAppPreferences that the issue describes as already prototyped, ensuring refresh failures no longer propagate errors when a stale cached value is available.

Poem

🐇 When the refresh fails and the network's slow,
I won't throw errors—I'll use what I know!
A stale old cache is better than none,
I'll log a debug and keep the stream spun.
No panics, no crashes—just calm bunny zen,
The tick keeps on ticking again and again! 🌿

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the main change: serving stale cached app info/preferences when refresh fails, which is the primary purpose of modifications across four files.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/qbit-slow-instance-resilience

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