fix: remove invalid expunge call on Pydantic FileModel
Files.get_file_by_id() returns a Pydantic FileModel, not an SQLAlchemy
ORM object. Calling db.expunge() on a Pydantic model fails with
UnmappedInstanceError since it lacks _sa_instance_state.
The expunge was also unnecessary because subsequent DB updates already
use fresh sessions via get_db() context manager.
Fixes#20925
b272ca5e88