refactor: improve code formatting and organization across multiple files

backend/app/repositories/users.py

@@ -17,11 +17,15 @@ class UserRepository(BaseRepository[User]):
         super().__init__(session)
 
     def get_by_username(self, username: str) -> User | None:
-        statement = sa.select(self.model).where(sa.func.lower(self.model.username) == username.lower())
+        statement = sa.select(self.model).where(
+            sa.func.lower(self.model.username) == username.lower()
+        )
         return self.session.scalar(statement)
 
     def list_recent(self, limit: int = 50) -> list[User]:
-        statement = sa.select(self.model).order_by(self.model.created_at.desc()).limit(limit)
+        statement = (
+            sa.select(self.model).order_by(self.model.created_at.desc()).limit(limit)
+        )
         return list(self.session.scalars(statement))
 
     def create(self, data: UserCreate) -> User: