feat: core schema redesign — dishes, structured recipes, cuisines, tags (iteration 7)

Replaces the flat JSONB-based recipe schema with a normalized relational model:

Schema (migrations consolidated to 001_initial_schema + 002_seed_data):
- New: dishes, dish_translations, dish_tags — canonical dish catalog
- New: cuisines, tags, dish_categories with _translations tables + full seed data
- New: recipe_ingredients, recipe_steps with _translations (replaces JSONB blobs)
- New: user_saved_recipes thin bookmark (drops saved_recipes + saved_recipe_translations)
- New: product_ingredients M2M table
- recipes: now a cooking variant of a dish (dish_id FK, no title/JSONB columns)
- recipe_translations: repurposed to per-language notes only
- products: mapping_id → primary_ingredient_id
- menu_items: recipe_id FK → recipes; adds dish_id
- meal_diary: adds dish_id, recipe_id → recipes, portion_g

Backend (Go):
- New packages: internal/cuisine, internal/tag, internal/dish (registry + handler + repo)
- New GET /cuisines, GET /tags (public), GET /dishes, GET /dishes/{id}, GET /recipes/{id}
- recipe, savedrecipe, menu, diary, product, ingredient packages updated for new schema

Flutter:
- New models: Cuisine, Tag; new providers: cuisineNamesProvider, tagNamesProvider
- recipe.dart: RecipeIngredient gains unit_code + effectiveUnit getter
- saved_recipe.dart: thin model, manual fromJson, computed nutrition getter
- diary_entry.dart: adds dishId, recipeId, portionG
- recipe_detail_screen.dart: localized cuisine/tag names via providers

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

backend/internal/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)
+}