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/domain/language/handler.go

@@ -0,0 +1,28 @@
+package language
+
+import (
+	"encoding/json"
+	"net/http"
+
+	"github.com/food-ai/backend/internal/infra/locale"
+)
+
+type languageItem struct {
+	Code       string `json:"code"`
+	NativeName string `json:"native_name"`
+	SortOrder  int    `json:"sort_order"`
+}
+
+// List handles GET /languages — returns the active language list loaded from DB.
+func List(w http.ResponseWriter, r *http.Request) {
+	items := make([]languageItem, 0, len(locale.Languages))
+	for i, l := range locale.Languages {
+		items = append(items, languageItem{
+			Code:       l.Code,
+			NativeName: l.NativeName,
+			SortOrder:  i + 1,
+		})
+	}
+	w.Header().Set("Content-Type", "application/json")
+	_ = json.NewEncoder(w).Encode(map[string]any{"languages": items})
+}