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/pyproject.toml

@@ -6,6 +6,9 @@ requires-python = ">=3.13"
 dependencies = [
     "aiomqtt>=2.0",
     "fastapi>=0.115",
+    # Podcast RSS feeds in the wild are full of small quirks (odd dates, missing
+    # namespaces); parsing them by hand invites silently dropping episodes.
+    "feedparser>=6.0",
     # Also what tests drive the ASGI app with - plain httpx is deprecated in favour of
     # this for exactly that. Runtime uses it to proxy the room page's Home Assistant
     # calls, so the long-lived token never has to leave the backend.
@@ -68,5 +71,5 @@ warn_unreachable = true
 [[tool.mypy.overrides]]
 # numpy ships its own types, but only when the `analysis` extra is installed - a plain
 # checkout must still type-check clean, so it needs the same treatment as librosa.
-module = ["vlc", "serial_asyncio", "ruamel.*", "librosa.*", "numpy"]
+module = ["vlc", "serial_asyncio", "ruamel.*", "librosa.*", "numpy", "feedparser"]
 ignore_missing_imports = true