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/middleware/auth_test.go

@@ -7,7 +7,7 @@ import (
 	"testing"
 	"time"
 
-	"github.com/food-ai/backend/internal/middleware"
+	"github.com/food-ai/backend/internal/infra/middleware"
 	"github.com/golang-jwt/jwt/v5"
 )
 

backend/tests/middleware/recovery_test.go

@@ -5,7 +5,7 @@ import (
 	"net/http/httptest"
 	"testing"
 
-	"github.com/food-ai/backend/internal/middleware"
+	"github.com/food-ai/backend/internal/infra/middleware"
 )
 
 func TestRecovery_NoPanic(t *testing.T) {

backend/tests/middleware/request_id_test.go

@@ -5,7 +5,7 @@ import (
 	"net/http/httptest"
 	"testing"
 
-	"github.com/food-ai/backend/internal/middleware"
+	"github.com/food-ai/backend/internal/infra/middleware"
 )
 
 func TestRequestID_GeneratesNew(t *testing.T) {