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/language.go

@@ -0,0 +1,18 @@
+package middleware
+
+import (
+	"net/http"
+
+	"github.com/food-ai/backend/internal/infra/locale"
+)
+
+// Language reads the Accept-Language request header, resolves the best
+// supported language via locale.Parse, and stores it in the request context.
+// Downstream handlers retrieve it with locale.FromContext.
+func Language(next http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		lang := locale.FromRequest(r)
+		ctx := locale.WithLang(r.Context(), lang)
+		next.ServeHTTP(w, r.WithContext(ctx))
+	})
+}