refactor: move Firebase implementation to adapters/firebase, drop pexels interface

- Create internal/adapters/firebase/auth.go with Auth, noopAuth, NewAuthOrNoop
  (renamed from FirebaseAuth, noopTokenVerifier, NewFirebaseAuthOrNoop)
- Reduce internal/auth/firebase.go to TokenVerifier interface only
- Remove PhotoSearcher interface from adapters/pexels (belongs to consumers)
- Update wire.go and wire_gen.go to use firebase.NewAuthOrNoop

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/internal/adapters/pexels/client.go

@@ -14,11 +14,6 @@ const (
 	defaultPlaceholder = "https://images.pexels.com/photos/1640777/pexels-photo-1640777.jpeg?auto=compress&cs=tinysrgb&w=1260&h=750"
 )
 
-// PhotoSearcher can search for a photo by text query.
-type PhotoSearcher interface {
-	SearchPhoto(ctx context.Context, query string) (string, error)
-}
-
 // Client is an HTTP client for the Pexels Photos API.
 type Client struct {
 	apiKey     string