Auto-download new podcast episodes from an RSS feed

A show folder under Kinderpodcasts/ opts in by containing a feed.txt marker
naming its RSS feed. A new PodcastFeedService polls every such feed every 6
hours (and once at startup), downloads any episode not already on disk using
the existing YYYYMMDD - Title.ext convention, and triggers the same
rescan-and-broadcast sequence "Bibliothek neu einlesen" already uses - now
shared via App.rescan_library() instead of duplicated. A show with no
feed.txt is untouched, so there is no new config section for this.

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

python-backend/README.md

@@ -165,6 +165,12 @@ Only files whose suffix is in `audio_extensions` are read, and dotfiles are skip
 a podcast downloader's `archive.json` and its half-finished `.download.tmp` never reach
 a playlist.
 
+Drop a `feed.txt` into a show folder (its first line the show's RSS feed URL) and the
+backend becomes that podcast downloader itself: every six hours it checks the feed and
+saves any episode not already on disk, named `YYYYMMDD - Title.ext` like a hand-placed
+one so it sorts and scans identically. A show with no `feed.txt` is untouched, exactly
+as before - the file is the opt-in, there is no separate setting for it.
+
 Each album carries three colours, pulled out of its cover art with Pillow (or
 synthesised from a hash of its id when it has none). The frontend paints cards with
 them and the LED strips run the first of them, so shelf and screen agree.