Fix concurrency issues and add seed limit enforcement

[exec]

This PR addresses several critical concurrency bugs and adds seed-limit enforcement for seeding torrents.

Summary

The changes fix race conditions in the app startup sequence, add persistent seed-limit tracking across engine sessions, improve RSS feed deduplication, and enhance the updater with better version comparison and AppImage support.

Key Changes

Concurrency & Startup Fixes:

• Add mutex-protected context initialization in App to handle launch args arriving before startup completes (e.g., Linux second-instance listener)
• Buffer pending launch args and drain them once the context exists, preventing nil-context panics
• Separate tick goroutines into their own cancellable context to ensure clean shutdown
• Add windowVisible flag to gate Wails event emission when the window is hidden

Seed Limit Enforcement:

• Add TotalUploaded/TotalDownloaded cumulative counters to TorrentRecord to survive engine session resets
• Implement CheckSeedLimits() to pause torrents exceeding ratio/duration limits using persisted totals
• Add sessionCounters tracking to compute deltas between observations
• Add comprehensive tests covering ratio-based pausing and counter persistence

RSS & Feed Improvements:

• Add RSSSeen persistence layer to track seen feed GUIDs across restarts (fixes re-adding old items on restart)
• Replace in-memory gofeed parser with persistent dedup via database
• Add rssBodyCap to limit feed body parsing to 10 MB
• Fix WatchFolder double-close panic by swapping stop/done channels before closing

Engine & Limiter Fixes:

• Add maxConnsPerTorrent atomic field to persist user's per-torrent connection cap across pause/resume cycles
• Add limiterRunning map to prevent spawning duplicate rate-limiter goroutines
• Add snapshotInflight and snapshotRetryAt to prevent goroutine leaks on persistent snapshot write failures

Updater Improvements:

• Add OnUpdaterConfigChange() callback to push channel changes to live GitHubSource without restart
• Add compareVersions() with proper semantic versioning and pre-release handling
• Fix AppImage installation by detecting APPIMAGE env var and swapping the runtime file instead of the squashfs mount
• Add permission check to CheckForUpdate() to gate outbound HTTP requests

Other Fixes:

• Add unixOrZero() helper to serialize zero timestamps as 0 instead of year-1 dates
• Fix ListTorrents error handling to use writeServiceErr for consistent HTTP status codes
• Add verifyPrivateDir() security check for /tmp fallback socket directory on Linux
• Fix Subscriber lifecycle to prevent blocking on done when Start() never ran
• Add Unsubscribe() to event bus to prevent dead channel leaks on server restarts
• Fix ScheduleEngine change detection to re-apply limits when edited rule is active
• Add rune-aware truncation for non-ASCII torrent names in notifications
• Add tracker add/remove routes to frontend HTTP transport
• Improve config validation to reject unknown YAML keys and validate port ranges
• Add session lifecycle tests for reconfigure vs. full disable transitions

Notable Implementation Details

• Launch args are buffered in pendingArgs slice under ctxMu lock, drained by startup() once context exists
• Seed limit checking uses delta computation: (current - lastObserved) + persisted = new total
• RSS dedup now survives restarts via rss_seen table with cascade delete on feed removal
• Rate limiter goroutines tracked in limiterRunning map to prevent duplicates on rapid limit changes
• Snapshot save failures no longer leak goroutines; snapshotRetryAt gates retry attempts

https://claude.ai/code/session_0112xsVQpsrg8TzRqPRqzbZ7