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/dish/handler.go

@@ -0,0 +1,67 @@
+package dish
+
+import (
+	"encoding/json"
+	"log/slog"
+	"net/http"
+
+	"github.com/go-chi/chi/v5"
+)
+
+// Handler handles HTTP requests for dishes.
+type Handler struct {
+	repo *Repository
+}
+
+// NewHandler creates a new Handler.
+func NewHandler(repo *Repository) *Handler {
+	return &Handler{repo: repo}
+}
+
+// List handles GET /dishes — returns all dishes (no recipe variants).
+func (h *Handler) List(w http.ResponseWriter, r *http.Request) {
+	dishes, err := h.repo.List(r.Context())
+	if err != nil {
+		slog.Error("list dishes", "err", err)
+		writeError(w, http.StatusInternalServerError, "failed to list dishes")
+		return
+	}
+	if dishes == nil {
+		dishes = []*Dish{}
+	}
+	writeJSON(w, http.StatusOK, map[string]any{"dishes": dishes})
+}
+
+// GetByID handles GET /dishes/{id} — returns a dish with all recipe variants.
+func (h *Handler) GetByID(w http.ResponseWriter, r *http.Request) {
+	id := chi.URLParam(r, "id")
+	dish, err := h.repo.GetByID(r.Context(), id)
+	if err != nil {
+		slog.Error("get dish", "id", id, "err", err)
+		writeError(w, http.StatusInternalServerError, "failed to get dish")
+		return
+	}
+	if dish == nil {
+		writeError(w, http.StatusNotFound, "dish not found")
+		return
+	}
+	writeJSON(w, http.StatusOK, dish)
+}
+
+// --- helpers ---
+
+type errorResponse struct {
+	Error string `json:"error"`
+}
+
+func writeError(w http.ResponseWriter, status int, msg string) {
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(status)
+	_ = json.NewEncoder(w).Encode(errorResponse{Error: msg})
+}
+
+func writeJSON(w http.ResponseWriter, status int, v any) {
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(status)
+	_ = json.NewEncoder(w).Encode(v)
+}