Keep only the newest 50 episodes of each podcast

A show that has published for years is unbounded: GEOlino Spezial alone is 358
episodes and 5.8 GB, and the device it syncs onto is a 30 GB SD card that also
holds the rest of the library. Nothing stopped the 6-hourly poll from eventually
filling it.

Cap each show's folder at general.podcast_episode_limit (default 50, null to
keep everything), pruning the oldest past that after each sync pass.

The same limit caps what is downloaded, and it has to be one number for both.
Prune to the newest N but keep fetching everything the feed offers, and every
poll would re-download exactly the episodes the previous one deleted - forever,
at full size, since missing_episodes() decides purely from what is on disk.
There is a test for that specific loop.

Pruning only touches files named the way this module names them
(YYYYMMDD - Title.ext), so feed.txt, folder.jpg, the failed-download record and
anything placed by hand are all left alone; a parse that fails means "not ours",
not "delete it". An episode's sidecar cover goes with it. A pass that only
deleted still reports a change, because the library needs the rescan just as
much as it does after a download.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

python-backend/config.yml.example

@@ -61,6 +61,19 @@ general:
   # a half-finished .tmp - is ignored.
   audio_extensions: [".mp3", ".ogg", ".oga", ".opus", ".flac", ".wav", ".m4a", ".aac"]
 
+  # How many episodes of each podcast show to keep, newest first. A show that has
+  # published for years grows without bound: GEOlino Spezial alone is 358 episodes and
+  # 5.8 GB, which will not sit next to the rest of a library on a Pi's SD card.
+  #
+  # The same number caps what gets downloaded, which is what makes the folder settle.
+  # Prune to the newest N but fetch everything the feed offers, and every poll would
+  # re-download the episodes the last one deleted.
+  #
+  # Lowering this DELETES the episodes that fall outside the window on the next poll,
+  # and an episode that has aged out of its feed cannot be fetched again. Use `null` to
+  # keep every episode and mind the free space yourself.
+  podcast_episode_limit: 50
+
   # The web front-end. Omit the whole section to run without it.
   #
   # There is no authentication: this is a device on a home network. The settings panel