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/infra/middleware/cors.go

@@ -0,0 +1,18 @@
+package middleware
+
+import (
+	"net/http"
+
+	"github.com/go-chi/cors"
+)
+
+func CORS(allowedOrigins []string) func(http.Handler) http.Handler {
+	return cors.Handler(cors.Options{
+		AllowedOrigins:   allowedOrigins,
+		AllowedMethods:   []string{"GET", "POST", "PUT", "PATCH", "DELETE", "OPTIONS"},
+		AllowedHeaders:   []string{"Authorization", "Content-Type", "X-Request-ID"},
+		ExposedHeaders:   []string{"X-Request-ID"},
+		AllowCredentials: true,
+		MaxAge:           300,
+	})
+}