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>

tests/test_lastfm.py

@@ -5,7 +5,7 @@ import importlib.util
 import json
 import sys
 from dataclasses import dataclass
-from unittest.mock import MagicMock, patch
+from unittest.mock import AsyncMock, MagicMock, patch
 
 # -- Load plugin module directly ---------------------------------------------
 
@@ -521,3 +521,142 @@ class TestCmdTags:
         with patch.object(_mod, "_get_top_tags", return_value=tags):
             asyncio.run(_mod.cmd_tags(bot, msg))
         assert any("Lateralus:" in r for r in bot.replied)
+
+
+# ---------------------------------------------------------------------------
+# TestParseTitle
+# ---------------------------------------------------------------------------
+
+
+class TestParseTitle:
+    def test_dash_separator(self):
+        assert _mod._parse_title("Tool - Lateralus") == ("Tool", "Lateralus")
+
+    def test_double_dash(self):
+        assert _mod._parse_title("Tool -- Lateralus") == ("Tool", "Lateralus")
+
+    def test_pipe_separator(self):
+        assert _mod._parse_title("Tool | Lateralus") == ("Tool", "Lateralus")
+
+    def test_tilde_separator(self):
+        assert _mod._parse_title("Tool ~ Lateralus") == ("Tool", "Lateralus")
+
+    def test_no_separator(self):
+        assert _mod._parse_title("Lateralus") == ("", "Lateralus")
+
+    def test_empty_string(self):
+        assert _mod._parse_title("") == ("", "")
+
+    def test_strips_whitespace(self):
+        assert _mod._parse_title("  Tool  -  Lateralus  ") == ("Tool", "Lateralus")
+
+    def test_first_separator_wins(self):
+        """Only the first matching separator is used."""
+        assert _mod._parse_title("A - B - C") == ("A", "B - C")
+
+    def test_dash_priority_over_pipe(self):
+        """Dash separator is tried before pipe."""
+        assert _mod._parse_title("A - B | C") == ("A", "B | C")
+
+
+# ---------------------------------------------------------------------------
+# TestDiscoverSimilar
+# ---------------------------------------------------------------------------
+
+
+class TestDiscoverSimilar:
+    def test_lastfm_path(self):
+        """Returns Last.fm result when API key + results available."""
+        bot = _FakeBot(api_key="test-key")
+        tracks = [{"name": "Found", "artist": {"name": "Band"}, "match": "0.9"}]
+        with patch.object(_mod, "_get_similar_tracks", return_value=tracks):
+            result = asyncio.run(
+                _mod.discover_similar(bot, "Tool - Lateralus"),
+            )
+        assert result == ("Band", "Found")
+
+    def test_lastfm_empty_falls_to_musicbrainz(self):
+        """Falls back to MusicBrainz when Last.fm returns nothing."""
+        bot = _FakeBot(api_key="test-key")
+        mb_picks = [{"artist": "MB Artist", "title": "MB Song"}]
+        with patch.object(_mod, "_get_similar_tracks", return_value=[]), \
+             patch("plugins._musicbrainz.mb_search_artist", return_value="mbid"), \
+             patch("plugins._musicbrainz.mb_artist_tags", return_value=["rock"]), \
+             patch("plugins._musicbrainz.mb_find_similar_recordings",
+                   return_value=mb_picks):
+            result = asyncio.run(
+                _mod.discover_similar(bot, "Tool - Lateralus"),
+            )
+        assert result == ("MB Artist", "MB Song")
+
+    def test_no_api_key_uses_musicbrainz(self):
+        """Skips Last.fm when no API key, goes straight to MusicBrainz."""
+        bot = _FakeBot(api_key="")
+        mb_picks = [{"artist": "MB Band", "title": "MB Track"}]
+        with patch.dict("os.environ", {}, clear=True), \
+             patch("plugins._musicbrainz.mb_search_artist", return_value="mbid"), \
+             patch("plugins._musicbrainz.mb_artist_tags", return_value=["rock"]), \
+             patch("plugins._musicbrainz.mb_find_similar_recordings",
+                   return_value=mb_picks):
+            result = asyncio.run(
+                _mod.discover_similar(bot, "Tool - Lateralus"),
+            )
+        assert result == ("MB Band", "MB Track")
+
+    def test_both_fail_returns_none(self):
+        """Returns None when both Last.fm and MusicBrainz fail."""
+        bot = _FakeBot(api_key="test-key")
+        with patch.object(_mod, "_get_similar_tracks", return_value=[]), \
+             patch("plugins._musicbrainz.mb_search_artist", return_value=None):
+            result = asyncio.run(
+                _mod.discover_similar(bot, "Tool - Lateralus"),
+            )
+        assert result is None
+
+    def test_no_artist_returns_none(self):
+        """Returns None when title has no artist component."""
+        bot = _FakeBot(api_key="test-key")
+        result = asyncio.run(
+            _mod.discover_similar(bot, "Lateralus"),
+        )
+        assert result is None
+
+    def test_musicbrainz_import_error_handled(self):
+        """Gracefully handles import error for _musicbrainz module."""
+        bot = _FakeBot(api_key="test-key")
+        with patch.object(_mod, "_get_similar_tracks", return_value=[]), \
+             patch.dict("sys.modules", {"plugins._musicbrainz": None}):
+            result = asyncio.run(
+                _mod.discover_similar(bot, "Tool - Lateralus"),
+            )
+        assert result is None
+
+    def test_lastfm_exception_falls_to_musicbrainz(self):
+        """Last.fm exception triggers MusicBrainz fallback."""
+        bot = _FakeBot(api_key="test-key")
+        mb_picks = [{"artist": "Fallback", "title": "Song"}]
+        with patch.object(_mod, "_get_similar_tracks",
+                          side_effect=Exception("API down")), \
+             patch("plugins._musicbrainz.mb_search_artist", return_value="mbid"), \
+             patch("plugins._musicbrainz.mb_artist_tags", return_value=["rock"]), \
+             patch("plugins._musicbrainz.mb_find_similar_recordings",
+                   return_value=mb_picks):
+            result = asyncio.run(
+                _mod.discover_similar(bot, "Tool - Lateralus"),
+            )
+        assert result == ("Fallback", "Song")
+
+    def test_lastfm_pick_missing_name_falls_to_musicbrainz(self):
+        """Falls to MB when Last.fm result has empty artist/title."""
+        bot = _FakeBot(api_key="test-key")
+        tracks = [{"name": "", "artist": {"name": ""}, "match": "0.9"}]
+        mb_picks = [{"artist": "MB", "title": "Track"}]
+        with patch.object(_mod, "_get_similar_tracks", return_value=tracks), \
+             patch("plugins._musicbrainz.mb_search_artist", return_value="mbid"), \
+             patch("plugins._musicbrainz.mb_artist_tags", return_value=["rock"]), \
+             patch("plugins._musicbrainz.mb_find_similar_recordings",
+                   return_value=mb_picks):
+            result = asyncio.run(
+                _mod.discover_similar(bot, "Tool - Lateralus"),
+            )
+        assert result == ("MB", "Track")