Add per-episode podcast cover art and fix nested album grouping

- podcast_feeds.py: fetch per-episode cover art from a feed's itunes:image
  (when it's genuinely distinct from the channel image) or, failing that,
  from the og:image on the episode's own linked page; extract audio from
  video-only enclosures via ffmpeg; match podcast-dl's filename convention
  (illegal characters become "_" instead of being dropped) so enabling
  feed.txt on an already-downloaded show doesn't re-download its back catalog
- scanner.py: _cover_for_episode now checks for a same-stem sidecar cover
  image before falling back to embedded ID3 art and the shared folder cover
- scanner.py/__init__.py: fixed a bug where an album folder nested one level
  deeper than usual (an age-range grouping folder, say) was mistaken for an
  empty album and skipped; added periodic progress logging for long scans
  and analysis passes

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

python-backend/tests/test_library.py

@@ -237,6 +237,22 @@ async def test_a_cover_file_is_found_and_used(config_dir: Path) -> None:
     assert album.colors[0] != colors_from_id(album.id)[0]
 
 
+async def test_an_episodes_sidecar_cover_wins_over_the_shows_shared_one(config_dir: Path) -> None:
+    from PIL import Image
+
+    # Downloaded by `podcast_feeds.py` when a feed has real per-episode art (see its
+    # `episode_cover_filename`) - same stem as the episode's audio file.
+    podcast = config_dir / "music" / "Kinderpodcasts" / "Wissen macht Ah"
+    Image.new("RGB", (32, 32), (10, 20, 30)).save(podcast / "cover.jpg")
+    Image.new("RGB", (32, 32), (200, 40, 30)).save(podcast / "20260101 - Neu.jpg")
+
+    library = await build(config_dir)
+
+    assert album_named(library, "Neu").cover == podcast / "20260101 - Neu.jpg"
+    # No sidecar for this one, so it still falls back to the show's shared cover.
+    assert album_named(library, "Alt").cover == podcast / "cover.jpg"
+
+
 # --------------------------------------------------------------------------- cache
 
 
@@ -359,6 +375,28 @@ def test_album_ids_are_stable_and_path_derived(tmp_path: Path) -> None:
     assert first != other
 
 
+async def test_an_album_nested_under_an_extra_grouping_folder_is_still_found(
+    config_dir: Path,
+) -> None:
+    """The bug this fixed: an artist folder that groups its books one level deeper than
+    usual (an age-range folder, say) has no audio directly in it, so the scanner used to
+    treat it as an empty album and skip the whole artist rather than looking further
+    down for the actual album folders."""
+    for index in range(2):
+        write_track(
+            config_dir / "music" / "Hörbücher" / "Petzi" / "ab 5" / "Petzi und der Wal"
+            / f"{index:02d} - teil.mp3",
+            title=f"Teil {index}",
+            album="Petzi und der Wal",
+            albumartist="Petzi",
+        )
+    library = await build(config_dir)
+
+    album = album_named(library, "Petzi und der Wal")
+    assert album.kind == "book"
+    assert len(album.tracks) == 2
+
+
 # -------------------------------------------------------------------- figure kinds