refactor: introduce adapter pattern for AI provider (OpenAI)

- Add internal/adapters/ai/types.go with neutral shared types
  (Recipe, DayPlan, RecognizedItem, IngredientClassification, etc.)
- Remove types from internal/adapters/openai/ — adapter now uses ai.*
- Define Recognizer interface in recognition package
- Define MenuGenerator interface in menu package
- Define RecipeGenerator interface in recommendation package
- Handler structs now hold interfaces, not *openai.Client
- Add wire.Bind entries for the three new interface bindings

To swap OpenAI for another provider: implement the three interfaces
using ai.* types and change the wire.Bind lines in cmd/server/wire.go.

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

backend/internal/menu/handler.go

@@ -10,8 +10,8 @@ import (
 	"sync"
 	"time"
 
+	"github.com/food-ai/backend/internal/adapters/ai"
 	"github.com/food-ai/backend/internal/dish"
-	"github.com/food-ai/backend/internal/adapters/openai"
 	"github.com/food-ai/backend/internal/infra/locale"
 	"github.com/food-ai/backend/internal/infra/middleware"
 	"github.com/food-ai/backend/internal/user"
@@ -38,10 +38,15 @@ type RecipeSaver interface {
 	Create(ctx context.Context, req dish.CreateRequest) (string, error)
 }
 
+// MenuGenerator generates a 7-day meal plan via an AI provider.
+type MenuGenerator interface {
+	GenerateMenu(ctx context.Context, req ai.MenuRequest) ([]ai.DayPlan, error)
+}
+
 // Handler handles menu and shopping-list endpoints.
 type Handler struct {
 	repo          *Repository
-	openaiClient  *openai.Client
+	menuGenerator MenuGenerator
 	pexels        PhotoSearcher
 	userLoader    UserLoader
 	productLister ProductLister
@@ -51,7 +56,7 @@ type Handler struct {
 // NewHandler creates a new Handler.
 func NewHandler(
 	repo *Repository,
-	openaiClient *openai.Client,
+	menuGenerator MenuGenerator,
 	pexels PhotoSearcher,
 	userLoader UserLoader,
 	productLister ProductLister,
@@ -59,7 +64,7 @@ func NewHandler(
 ) *Handler {
 	return &Handler{
 		repo:          repo,
-		openaiClient:  openaiClient,
+		menuGenerator: menuGenerator,
 		pexels:        pexels,
 		userLoader:    userLoader,
 		productLister: productLister,
@@ -137,7 +142,7 @@ func (h *Handler) GenerateMenu(w http.ResponseWriter, r *http.Request) {
 	}
 
 	// Generate 7-day plan via Gemini.
-	days, err := h.openaiClient.GenerateMenu(r.Context(), menuReq)
+	days, err := h.menuGenerator.GenerateMenu(r.Context(), menuReq)
 	if err != nil {
 		slog.Error("generate menu", "user_id", userID, "err", err)
 		writeError(w, http.StatusServiceUnavailable, "menu generation failed, please try again")
@@ -449,8 +454,8 @@ type userPreferences struct {
 	Restrictions []string `json:"restrictions"`
 }
 
-func buildMenuRequest(u *user.User, lang string) openai.MenuRequest {
-	req := openai.MenuRequest{DailyCalories: 2000, Lang: lang}
+func buildMenuRequest(u *user.User, lang string) ai.MenuRequest {
+	req := ai.MenuRequest{DailyCalories: 2000, Lang: lang}
 	if u.Goal != nil {
 		req.UserGoal = *u.Goal
 	}
@@ -468,7 +473,7 @@ func buildMenuRequest(u *user.User, lang string) openai.MenuRequest {
 }
 
 // recipeToCreateRequest converts a Gemini recipe to a dish.CreateRequest.
-func recipeToCreateRequest(r openai.Recipe) dish.CreateRequest {
+func recipeToCreateRequest(r ai.Recipe) dish.CreateRequest {
 	cr := dish.CreateRequest{
 		Name:        r.Title,
 		Description: r.Description,