refactor: migrate DI to Wire, replace startup registries with on-demand DB queries

- Add google/wire; generate wire_gen.go from wire.go injector
- Move all constructor wiring out of main.go into providers.go / wire.go
- Export recognition.IngredientRepository (was unexported, blocked Wire binding)
- Delete units/cuisine/tag registry.go files (global state + unused NameFor helpers)
- Replace List handlers with NewListHandler(pool) using COALESCE SQL queries
- Remove units/cuisine/tag LoadFromDB from server startup; locale.LoadFromDB kept
  (locale.Supported is used by language middleware on every request)

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

backend/internal/recognition/handler.go

@@ -13,8 +13,8 @@ import (
 	"github.com/food-ai/backend/internal/middleware"
 )
 
-// ingredientRepo is the subset of ingredient.Repository used by this handler.
-type ingredientRepo interface {
+// IngredientRepository is the subset of ingredient.Repository used by this handler.
+type IngredientRepository interface {
 	FuzzyMatch(ctx context.Context, name string) (*ingredient.IngredientMapping, error)
 	Upsert(ctx context.Context, m *ingredient.IngredientMapping) (*ingredient.IngredientMapping, error)
 	UpsertTranslation(ctx context.Context, id, lang, name string) error
@@ -24,11 +24,11 @@ type ingredientRepo interface {
 // Handler handles POST /ai/* recognition endpoints.
 type Handler struct {
 	gemini         *gemini.Client
-	ingredientRepo ingredientRepo
+	ingredientRepo IngredientRepository
 }
 
 // NewHandler creates a new Handler.
-func NewHandler(geminiClient *gemini.Client, repo ingredientRepo) *Handler {
+func NewHandler(geminiClient *gemini.Client, repo IngredientRepository) *Handler {
 	return &Handler{gemini: geminiClient, ingredientRepo: repo}
 }