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/recommendation/handler.go

@@ -8,7 +8,7 @@ import (
 	"strconv"
 	"sync"
 
-	"github.com/food-ai/backend/internal/adapters/openai"
+	"github.com/food-ai/backend/internal/adapters/ai"
 	"github.com/food-ai/backend/internal/infra/locale"
 	"github.com/food-ai/backend/internal/infra/middleware"
 	"github.com/food-ai/backend/internal/user"
@@ -35,21 +35,26 @@ type userPreferences struct {
 	Restrictions []string `json:"restrictions"`
 }
 
+// RecipeGenerator generates recipe recommendations via an AI provider.
+type RecipeGenerator interface {
+	GenerateRecipes(ctx context.Context, req ai.RecipeRequest) ([]ai.Recipe, error)
+}
+
 // Handler handles GET /recommendations.
 type Handler struct {
-	openaiClient  *openai.Client
-	pexels        PhotoSearcher
-	userLoader    UserLoader
-	productLister ProductLister
+	recipeGenerator RecipeGenerator
+	pexels          PhotoSearcher
+	userLoader      UserLoader
+	productLister   ProductLister
 }
 
 // NewHandler creates a new Handler.
-func NewHandler(openaiClient *openai.Client, pexels PhotoSearcher, userLoader UserLoader, productLister ProductLister) *Handler {
+func NewHandler(recipeGenerator RecipeGenerator, pexels PhotoSearcher, userLoader UserLoader, productLister ProductLister) *Handler {
 	return &Handler{
-		openaiClient:  openaiClient,
-		pexels:        pexels,
-		userLoader:    userLoader,
-		productLister: productLister,
+		recipeGenerator: recipeGenerator,
+		pexels:          pexels,
+		userLoader:      userLoader,
+		productLister:   productLister,
 	}
 }
 
@@ -84,7 +89,7 @@ func (h *Handler) GetRecommendations(w http.ResponseWriter, r *http.Request) {
 		slog.Warn("load products for recommendations", "user_id", userID, "err", err)
 	}
 
-	recipes, err := h.openaiClient.GenerateRecipes(r.Context(), req)
+	recipes, err := h.recipeGenerator.GenerateRecipes(r.Context(), req)
 	if err != nil {
 		slog.Error("generate recipes", "user_id", userID, "err", err)
 		writeErrorJSON(w, http.StatusServiceUnavailable, "recipe generation failed, please try again")
@@ -109,8 +114,8 @@ func (h *Handler) GetRecommendations(w http.ResponseWriter, r *http.Request) {
 	writeJSON(w, http.StatusOK, recipes)
 }
 
-func buildRecipeRequest(u *user.User, count int, lang string) openai.RecipeRequest {
-	req := openai.RecipeRequest{
+func buildRecipeRequest(u *user.User, count int, lang string) ai.RecipeRequest {
+	req := ai.RecipeRequest{
 		Count:         count,
 		DailyCalories: 2000, // sensible default
 		Lang:          lang,