Fix goroutine leaks in sync queue processor and connection manager

Critical fixes to prevent goroutine leaks during application shutdown:

1. Sync Queue Processor:
   - Changed StartCleanupTask() to return context.CancelFunc
   - Modified to accept and watch cancellable context
   - Added queue context/cancel to Handler struct
   - Created StartBackgroundTasks() method for main handler instance
   - Cancel queue processor during shutdown in StopScheduler()

2. Connection Manager:
   - Modified StartCleanupTask() to use cancellable context
   - Returns cancel function that can be called during shutdown
   - Goroutine now properly exits when context is cancelled

3. Handler Lifecycle:
   - Added StartBackgroundTasks() to Handler
   - Only main handler instance starts background goroutines
   - Temporary handler instances (library/sync routes) don't start tasks
   - StopScheduler() now properly shuts down all background goroutines

4. Router Integration:
   - Updated SetupRoutes to accept queueProcessor parameter
   - Main scanner handler starts background tasks after creation
   - Library and sync route handlers don't start duplicate tasks

Impact:
- Fixes 2 major goroutine leaks (queue processor + connection cleanup)
- Application now properly shuts down all goroutines on exit
- No more resource leaks from long-running goroutines
- Test added to detect future goroutine regressions

Test: TestGoroutineCleanup verifies background services can be stopped.

internal/router/sync.go

@@ -13,7 +13,7 @@ func registerSyncRoutes(cfg *Config) {
 	protected := e.Group("/api", jwtMiddleware)
 
 	// Create handler for sync-specific routes
-	h := handlers.NewHandler(cfg.Queries, cfg.ConnManager)
+	h := handlers.NewHandler(cfg.Queries, cfg.ConnManager, cfg.QueueProcessor)
 
 	// Book matching and unlinked book resolution routes
 	sync := protected.Group("/sync")