refactor: restructure internal/ into adapters/, infra/, and app layers

- internal/gemini/ → internal/adapters/openai/ (renamed package to openai)
- internal/pexels/ → internal/adapters/pexels/
- internal/config/   → internal/infra/config/
- internal/database/ → internal/infra/database/
- internal/locale/   → internal/infra/locale/
- internal/middleware/ → internal/infra/middleware/
- internal/server/   → internal/infra/server/

All import paths and call sites updated accordingly.

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

backend/internal/menu/handler.go

@@ -11,9 +11,9 @@ import (
 	"time"
 
 	"github.com/food-ai/backend/internal/dish"
-	"github.com/food-ai/backend/internal/gemini"
-	"github.com/food-ai/backend/internal/locale"
-	"github.com/food-ai/backend/internal/middleware"
+	"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"
 	"github.com/go-chi/chi/v5"
 )
@@ -41,7 +41,7 @@ type RecipeSaver interface {
 // Handler handles menu and shopping-list endpoints.
 type Handler struct {
 	repo          *Repository
-	gemini        *gemini.Client
+	openaiClient  *openai.Client
 	pexels        PhotoSearcher
 	userLoader    UserLoader
 	productLister ProductLister
@@ -51,7 +51,7 @@ type Handler struct {
 // NewHandler creates a new Handler.
 func NewHandler(
 	repo *Repository,
-	geminiClient *gemini.Client,
+	openaiClient *openai.Client,
 	pexels PhotoSearcher,
 	userLoader UserLoader,
 	productLister ProductLister,
@@ -59,7 +59,7 @@ func NewHandler(
 ) *Handler {
 	return &Handler{
 		repo:          repo,
-		gemini:        geminiClient,
+		openaiClient:  openaiClient,
 		pexels:        pexels,
 		userLoader:    userLoader,
 		productLister: productLister,
@@ -137,7 +137,7 @@ func (h *Handler) GenerateMenu(w http.ResponseWriter, r *http.Request) {
 	}
 
 	// Generate 7-day plan via Gemini.
-	days, err := h.gemini.GenerateMenu(r.Context(), menuReq)
+	days, err := h.openaiClient.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 +449,8 @@ type userPreferences struct {
 	Restrictions []string `json:"restrictions"`
 }
 
-func buildMenuRequest(u *user.User, lang string) gemini.MenuRequest {
-	req := gemini.MenuRequest{DailyCalories: 2000, Lang: lang}
+func buildMenuRequest(u *user.User, lang string) openai.MenuRequest {
+	req := openai.MenuRequest{DailyCalories: 2000, Lang: lang}
 	if u.Goal != nil {
 		req.UserGoal = *u.Goal
 	}
@@ -468,7 +468,7 @@ func buildMenuRequest(u *user.User, lang string) gemini.MenuRequest {
 }
 
 // recipeToCreateRequest converts a Gemini recipe to a dish.CreateRequest.
-func recipeToCreateRequest(r gemini.Recipe) dish.CreateRequest {
+func recipeToCreateRequest(r openai.Recipe) dish.CreateRequest {
 	cr := dish.CreateRequest{
 		Name:        r.Title,
 		Description: r.Description,