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/cmd/server/wire.go

@@ -6,6 +6,7 @@ import (
 	"net/http"
 
 	"github.com/food-ai/backend/internal/auth"
+	"github.com/food-ai/backend/internal/adapters/firebase"
 	"github.com/food-ai/backend/internal/infra/config"
 	"github.com/food-ai/backend/internal/diary"
 	"github.com/food-ai/backend/internal/dish"
@@ -37,7 +38,7 @@ func initRouter(appConfig *config.Config, pool *pgxpool.Pool) (http.Handler, err
 		newFirebaseCredentialsFile,
 
 		// Auth
-		auth.NewFirebaseAuthOrNoop,
+		firebase.NewAuthOrNoop,
 		newJWTManager,
 		newJWTAdapter,
 		newAuthMiddleware,