fix(admin-users): restore missing @router.get decorator on get_user endpoint

The @router.get("/{user_id:path}") decorator was accidentally dropped from the
get_user function when the /local/... routes were inserted above it in the
previous PR. Without the decorator the function was never registered as a GET
handler, so GET /api/admin/users/<id> matched the PUT/DELETE catch-all routes
and Starlette correctly returned 405 Method Not Allowed instead of 200/403.

Adding the decorator back restores the GET endpoint and fixes the 5 tests that
were failing with 405.

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

app/api/admin_users.py

@@ -502,6 +502,7 @@ def admin_set_password(
     return {"updated": True, "email": user.email}
 
 
+@router.get("/{user_id:path}", summary="Get details for a single user")
 def get_user(user_id: str, db: DbSession, _admin: AdminUser) -> dict[str, Any]:
     """Return profile and document statistics for a specific user."""
     doc_count = db.query(func.count(FileRecord.id)).filter(FileRecord.owner_id == user_id).scalar() or 0