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/recommendation/handler.go

@@ -8,9 +8,9 @@ import (
 	"strconv"
 	"sync"
 
-	"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"
 )
 
@@ -37,16 +37,16 @@ type userPreferences struct {
 
 // Handler handles GET /recommendations.
 type Handler struct {
-	gemini        *gemini.Client
+	openaiClient  *openai.Client
 	pexels        PhotoSearcher
 	userLoader    UserLoader
 	productLister ProductLister
 }
 
 // NewHandler creates a new Handler.
-func NewHandler(geminiClient *gemini.Client, pexels PhotoSearcher, userLoader UserLoader, productLister ProductLister) *Handler {
+func NewHandler(openaiClient *openai.Client, pexels PhotoSearcher, userLoader UserLoader, productLister ProductLister) *Handler {
 	return &Handler{
-		gemini:        geminiClient,
+		openaiClient:  openaiClient,
 		pexels:        pexels,
 		userLoader:    userLoader,
 		productLister: productLister,
@@ -84,7 +84,7 @@ func (h *Handler) GetRecommendations(w http.ResponseWriter, r *http.Request) {
 		slog.Warn("load products for recommendations", "user_id", userID, "err", err)
 	}
 
-	recipes, err := h.gemini.GenerateRecipes(r.Context(), req)
+	recipes, err := h.openaiClient.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 +109,8 @@ func (h *Handler) GetRecommendations(w http.ResponseWriter, r *http.Request) {
 	writeJSON(w, http.StatusOK, recipes)
 }
 
-func buildRecipeRequest(u *user.User, count int, lang string) gemini.RecipeRequest {
-	req := gemini.RecipeRequest{
+func buildRecipeRequest(u *user.User, count int, lang string) openai.RecipeRequest {
+	req := openai.RecipeRequest{
 		Count:         count,
 		DailyCalories: 2000, // sensible default
 		Lang:          lang,