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>

client/lib/shared/models/recipe.dart

@@ -59,14 +59,25 @@ class Recipe {
 class RecipeIngredient {
   final String name;
   final double amount;
+
+  /// Unit from Gemini recommendations (free-form string).
+  @JsonKey(defaultValue: '')
   final String unit;
 
+  /// Unit code from the DB-backed saved recipes endpoint.
+  @JsonKey(name: 'unit_code')
+  final String? unitCode;
+
   const RecipeIngredient({
     required this.name,
     required this.amount,
-    required this.unit,
+    this.unit = '',
+    this.unitCode,
   });
 
+  /// Returns the best available unit identifier for display / lookup.
+  String get effectiveUnit => unitCode ?? unit;
+
   factory RecipeIngredient.fromJson(Map<String, dynamic> json) =>
       _$RecipeIngredientFromJson(json);
   Map<String, dynamic> toJson() => _$RecipeIngredientToJson(this);