feat(auth): implement user authentication with JWT and NextAuth

- Added user registration and login functionality with JWT authentication.
- Created auth controller, service, and module in the backend.
- Implemented user model and user products management.
- Integrated NextAuth for session management on the frontend.
- Added middleware for protecting routes and handling public access.
- Updated frontend API routes to include authorization headers.
- Enhanced recipe and user product models to support ownership and visibility.
- Created registration and login pages in the frontend.
- Added necessary types for NextAuth session management.

frontend/lib/auth-headers.ts

@@ -0,0 +1,11 @@
+import { auth } from '../../auth';
+
+/**
+ * Returnerar Authorization-header med JWT från sessionen.
+ * Används i alla server-side API-proxy-routes.
+ */
+export async function getAuthHeaders(): Promise<Record<string, string>> {
+  const session = await auth();
+  if (!session?.accessToken) return {};
+  return { Authorization: `Bearer ${session.accessToken}` };
+}