feat: auto-discover similar tracks during autoplay via Last.fm/MusicBrainz

Every Nth autoplay pick (configurable via discover_ratio), query Last.fm
for similar tracks. When Last.fm has no key or returns nothing, fall back
to MusicBrainz tag-based recording search (no API key needed). Discovered
tracks are resolved via yt-dlp and deduplicated within the session. If
discovery fails, the kept-deck shuffle continues as before.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

plugins/lastfm.py

@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+import asyncio
 import json
 import logging
 import os
@@ -91,6 +92,19 @@ def _search_track(api_key: str, query: str,
 # -- Metadata extraction -----------------------------------------------------
 
 
+def _parse_title(raw_title: str) -> tuple[str, str]:
+    """Split a raw track title into (artist, title).
+
+    Tries common separators: `` - ``, `` -- ``, `` | ``, `` ~ ``.
+    Returns ``("", raw_title)`` if no separator is found.
+    """
+    for sep in (" - ", " -- ", " | ", " ~ "):
+        if sep in raw_title:
+            parts = raw_title.split(sep, 1)
+            return (parts[0].strip(), parts[1].strip())
+    return ("", raw_title)
+
+
 def _current_meta(bot) -> tuple[str, str]:
     """Extract artist and title from the currently playing track.
 
@@ -103,15 +117,60 @@ def _current_meta(bot) -> tuple[str, str]:
     if current is None:
         return ("", "")
     raw_title = current.title or ""
+    return _parse_title(raw_title)
 
-    # Try common "Artist - Title" patterns
-    for sep in (" - ", " -- ", " | ", " ~ "):
-        if sep in raw_title:
-            parts = raw_title.split(sep, 1)
-            return (parts[0].strip(), parts[1].strip())
 
-    # No separator -- treat whole thing as a search query
-    return ("", raw_title)
+# -- Discovery orchestrator --------------------------------------------------
+
+
+async def discover_similar(bot, last_track_title: str) -> tuple[str, str] | None:
+    """Find a similar track via Last.fm or MusicBrainz fallback.
+
+    Returns ``(artist, title)`` or ``None`` if nothing found.
+    """
+    artist, title = _parse_title(last_track_title)
+    loop = asyncio.get_running_loop()
+
+    # -- Last.fm path --
+    api_key = _get_api_key(bot)
+    if api_key and artist:
+        try:
+            similar = await loop.run_in_executor(
+                None, _get_similar_tracks, api_key, artist, title, 20,
+            )
+            if similar:
+                pick = random.choice(similar)
+                pick_artist = pick.get("artist", {}).get("name", "")
+                pick_title = pick.get("name", "")
+                if pick_artist and pick_title:
+                    return (pick_artist, pick_title)
+        except Exception:
+            log.warning("lastfm: discover via Last.fm failed", exc_info=True)
+
+    # -- MusicBrainz fallback --
+    if artist:
+        try:
+            from plugins._musicbrainz import (
+                mb_artist_tags,
+                mb_find_similar_recordings,
+                mb_search_artist,
+            )
+
+            mbid = await loop.run_in_executor(None, mb_search_artist, artist)
+            if mbid:
+                tags = await loop.run_in_executor(None, mb_artist_tags, mbid)
+                if tags:
+                    picks = await loop.run_in_executor(
+                        None, mb_find_similar_recordings, artist, tags, 20,
+                    )
+                    if picks:
+                        pick = random.choice(picks)
+                        return (pick["artist"], pick["title"])
+        except Exception:
+            log.warning("lastfm: discover via MusicBrainz failed",
+                        exc_info=True)
+
+    return None
 
 
 # -- Formatting --------------------------------------------------------------