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/tests/user/repository_integration_test.go

@@ -9,7 +9,7 @@ import (
 	"time"
 
 	"github.com/food-ai/backend/internal/testutil"
-	"github.com/food-ai/backend/internal/user"
+	"github.com/food-ai/backend/internal/domain/user"
 )
 
 func TestRepository_UpsertByFirebaseUID_Insert(t *testing.T) {