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.go

@@ -6,14 +6,14 @@ import (
 	"net/http"
 
 	"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"
 	"github.com/food-ai/backend/internal/ingredient"
 	"github.com/food-ai/backend/internal/menu"
-	"github.com/food-ai/backend/internal/middleware"
-	"github.com/food-ai/backend/internal/pexels"
+	"github.com/food-ai/backend/internal/infra/middleware"
+	"github.com/food-ai/backend/internal/adapters/pexels"
 	"github.com/food-ai/backend/internal/product"
 	"github.com/food-ai/backend/internal/recipe"
 	"github.com/food-ai/backend/internal/recognition"
@@ -27,7 +27,7 @@ import (
 func initRouter(appConfig *config.Config, pool *pgxpool.Pool) (http.Handler, error) {
 	wire.Build(
 		// Config extractors
-		newGeminiAPIKey,
+		newOpenAIAPIKey,
 		newPexelsAPIKey,
 		newJWTSecret,
 		newJWTAccessDuration,
@@ -49,7 +49,7 @@ func initRouter(appConfig *config.Config, pool *pgxpool.Pool) (http.Handler, err
 		user.NewHandler,
 
 		// External clients
-		newGeminiClient,
+		newOpenAIClient,
 		newPexelsClient,
 
 		// Ingredient