ingest: bias yt-dlp picks towards a discography's newest releases

Bandcamp label/artist pages list releases newest-first, so give the first
YTDLP_RECENT_COUNT entries a YTDLP_RECENT_BOOST multiplier when picking,
leaning the radio towards fresh music. Flat extraction carries no dates, so
list position is used as the recency proxy rather than a real date lookup.

The boost is gated on the source URL being a bandcamp discography listing:
single /album/ and /track/ pages (whether a URL-file line or one the pick
recursed into) list tracks in track order, not by recency, and non-bandcamp
sources have unverified ordering, so all of those keep a uniform pick.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -91,6 +91,14 @@ Add one URL per line: a single track, or a playlist/album/label/artist page to
 pick from. The file is mounted read-only, so you can edit it without rebuilding.
 A missing file just disables the yt-dlp source.
 
+To favour fresh music, set `RADIEO_YTDLP_RECENT_BOOST` (in `.env`) above `1.0`:
+when picking from a Bandcamp label/artist/discography page — which lists
+releases newest-first — the newest `RADIEO_YTDLP_RECENT_COUNT` releases (default
+`5`) get that multiplier on their odds (e.g. `2.0` makes them twice as likely).
+The default `1.0` disables it. The boost only applies to such discography
+listings; single `/album/` and `/track/` pages and non-Bandcamp sources keep a
+uniform pick.
+
 ### 4. ListenBrainz suggestions
 
 Point `RADIEO_LISTENBRAINZ_URL` (in `.env`) at your recommendations syndication