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/cmd/server/wire_gen.go

@@ -8,7 +8,7 @@ package main
 
 import (
 	"github.com/food-ai/backend/internal/auth"
-	"github.com/food-ai/backend/internal/config"
+	"github.com/food-ai/backend/internal/infra/config"
 	"github.com/food-ai/backend/internal/diary"
 	"github.com/food-ai/backend/internal/dish"
 	"github.com/food-ai/backend/internal/home"
@@ -41,8 +41,8 @@ func initRouter(appConfig *config.Config, pool *pgxpool.Pool) (http.Handler, err
 	handler := auth.NewHandler(service)
 	userService := user.NewService(repository)
 	userHandler := user.NewHandler(userService)
-	mainGeminiAPIKey := newGeminiAPIKey(appConfig)
-	client := newGeminiClient(mainGeminiAPIKey)
+	mainGeminiAPIKey := newOpenAIAPIKey(appConfig)
+	client := newOpenAIClient(mainGeminiAPIKey)
 	mainPexelsAPIKey := newPexelsAPIKey(appConfig)
 	pexelsClient := newPexelsClient(mainPexelsAPIKey)
 	productRepository := product.NewRepository(pool)