chore(router): remove unused CollectionHandler from config

Remove the CollectionHandler field from router.Config struct and its
initialization in main.go. This field was never used - collections are
registered directly in handlers.SetupRoutes() where a CollectionHandler
is created locally.

Changes:
- Remove CollectionHandler field from internal/router/router.go Config
- Remove CollectionHandler: nil line from cmd/server/main.go

This cleans up dead code from the router refactoring. Collections
continue to work correctly as they are registered in SetupRoutes().

Related: Router refactoring completion

internal/router/router.go

@@ -43,7 +43,6 @@ type Config struct {
 	ConflictHandler      *handlers.ConflictHandler
 	AnalyticsHandler     *handlers.AnalyticsHandler
 	QueueHandler         *handlers.QueueHandler
-	CollectionHandler    *handlers.CollectionHandler
 	OPDSHandler          *handlers.OPDSHandler
 	ConnManager          *sync.ConnectionManager
 	QueueProcessor       *sync.SyncQueueProcessor