Backend returns 500 on GET /v1/users/{display_name} when display name contains a space

## Bug The `GET /v1/users/{user_id}` endpoint returns **500 Internal Server Error** when the `user_id` path parameter is a display name containing a space (e.g., `E2E Tester`). ## Steps to reproduce 1. Create a user and set their `display_name` to `E2E Tester` 2. Call `GET /v1/users/E2E%20Tester` with a valid Bearer token 3. The backend returns `500 Internal Server Error` ## Expected behavior The endpoint should return the user profile, as documented in the OpenAPI spec (example: `Jane Doe`). ## Actual behavior | Request | Result | |---------|--------| | `GET /v1/users/E2E%20Tester` | **500 Internal Server Error** | | `GET /v1/users/me` | 200 OK | | `GET /v1/users/25` (numeric ID) | 200 OK | | `GET /v1/users/E2ETester` (wrong name) | 404 Not Found | ## Impact The `/@displayname` public profile page in the frontend relies on this endpoint. Any user with a space in their display name cannot have their public profile viewed by others.