refactor: introduce internal/domain/ layer, rename model.go → entity.go

Move all business-logic packages from internal/ root into internal/domain/:
  auth, cuisine, diary, dish, home, ingredient, language, menu, product,
  recipe, recognition, recommendation, savedrecipe, tag, units, user

Rename model.go → entity.go in packages that hold domain entities:
  diary, dish, home, ingredient, menu, product, recipe, savedrecipe, user

Update all import paths accordingly (adapters, infra/server, cmd/server,
tests). No logic changes.

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

backend/internal/adapters/firebase/auth.go

@@ -11,7 +11,7 @@ import (
 	firebaseAuth "firebase.google.com/go/v4/auth"
 	"google.golang.org/api/option"
 
-	"github.com/food-ai/backend/internal/auth"
+	"github.com/food-ai/backend/internal/domain/auth"
 )
 
 // Auth is the Firebase-backed implementation of auth.TokenVerifier.