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/tests/ingredient/repository_integration_test.go

@@ -7,7 +7,7 @@ import (
 	"testing"
 
 	"github.com/food-ai/backend/internal/ingredient"
-	"github.com/food-ai/backend/internal/locale"
+	"github.com/food-ai/backend/internal/infra/locale"
 	"github.com/food-ai/backend/internal/testutil"
 )