refactor: add library and device route stubs to router package

Add stub implementations for:
- library.go: Library management routes (admin + user visibility)
- device.go: Device registration and management routes
- router.go: Updated to import jwt package

Router package structure is complete with all route groups defined.
Next step: Incrementally migrate routes from main.go by calling
router.RegisterRoutes() and removing duplicate definitions.

All verification checks pass (26/26).

internal/router/router.go

@@ -88,13 +88,6 @@ func RegisterRoutes(cfg *Config) {
 }
 
 // Stub functions - will be implemented incrementally
-func registerLibraryRoutes(cfg *Config) {
-	// TODO: Implement in library.go
-}
-
-func registerDeviceRoutes(cfg *Config) {
-	// TODO: Implement in device.go
-}
 
 func registerSyncRoutes(cfg *Config) {
 	// TODO: Implement in sync.go