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

cmd/server/main.go

@@ -147,7 +147,6 @@ func main() {
 		ConflictHandler:      conflictHandler,
 		AnalyticsHandler:     analyticsHandler,
 		QueueHandler:         queueHandler,
-		CollectionHandler:    nil, // TODO: Initialize collection handler
 		OPDSHandler:          opdsHandler,
 		ConnManager:          connManager,
 		QueueProcessor:       queueProcessor,