Improve episode cleanup routine

[mm503]

Hi @mxpv ,

I did some manual cleanup of downloaded episodes and that broke the consistency between the DB and reality.

So I updated the cleanup logic to be more tolerant to these situations - and remove file references from the DB if the file is no longer present.

Example of how it worked in my setup:

time="2025-08-20T15:44:44Z" level=info msg="running cleaner" count=20 feed_id=world_sci_festival
time="2025-08-20T15:44:44Z" level=info msg="deleting \"Infinite Beginnings? Time in Cutting Edge Cosmology\"" episode_id=_DMbOtpRSk0 feed_id=world_sci_festival
time="2025-08-20T15:44:44Z" level=info msg="episode was not found - file does not exist" episode_id=_DMbOtpRSk0 feed_id=world_sci_festival
time="2025-08-20T15:44:44Z" level=info msg="deleting \"Consciousness, Free Will, and Psychedelics: Exploring Mysteries of the Mind\"" episode_id=C9bqa8hRonE feed_id=world_sci_festival
time="2025-08-20T15:44:44Z" level=info msg="episode was not found - file does not exist" episode_id=C9bqa8hRonE feed_id=world_sci_festival
time="2025-08-20T15:44:44Z" level=info msg="deleting \"Straight Talk on Quantum Computing\"" episode_id=AVYRW9Qdp7Q feed_id=world_sci_festival
time="2025-08-20T15:44:44Z" level=info msg="episode was not found - file does not exist" episode_id=AVYRW9Qdp7Q feed_id=world_sci_festival
...

(Consequent runs were clean)

[mxpv]

Thanks!