feat: add MusicBrainz fallback to !similar and !tags commands

Remove early return on missing Last.fm API key. Both commands now
fall back to MusicBrainz (mb_search_artist -> mb_artist_tags ->
mb_find_similar_recordings) when no API key is configured or when
Last.fm returns empty results. Same pattern used by discover_similar.

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

tests/test_lastfm.py

@@ -339,12 +339,56 @@ class TestFmtMatch:
 
 
 class TestCmdSimilar:
-    def test_no_api_key(self):
+    def test_no_api_key_mb_fallback(self):
+        """No API key falls back to MusicBrainz for similar results."""
         bot = _FakeBot(api_key="")
         msg = _Msg(text="!similar Tool")
-        with patch.dict("os.environ", {}, clear=True):
+        mb_picks = [{"artist": "MB Artist", "title": "MB Song"}]
+        with patch.dict("os.environ", {}, clear=True), \
+             patch("plugins._musicbrainz.mb_search_artist",
+                   return_value="mbid-123"), \
+             patch("plugins._musicbrainz.mb_artist_tags",
+                   return_value=["rock", "metal"]), \
+             patch("plugins._musicbrainz.mb_find_similar_recordings",
+                   return_value=mb_picks):
             asyncio.run(_mod.cmd_similar(bot, msg))
-        assert any("not configured" in r for r in bot.replied)
+        assert any("Similar to Tool" in r for r in bot.replied)
+        assert any("MB Artist" in r for r in bot.replied)
+
+    def test_no_api_key_mb_no_results(self):
+        """No API key + MusicBrainz returns nothing shows 'no similar'."""
+        bot = _FakeBot(api_key="")
+        msg = _Msg(text="!similar Tool")
+        with patch.dict("os.environ", {}, clear=True), \
+             patch("plugins._musicbrainz.mb_search_artist",
+                   return_value=None):
+            asyncio.run(_mod.cmd_similar(bot, msg))
+        assert any("No similar artists" in r for r in bot.replied)
+
+    def test_no_api_key_play_mode(self):
+        """No API key + play mode delegates to cmd_play via MB results."""
+        bot = _FakeBot(api_key="")
+        msg = _Msg(text="!similar play Tool")
+        mb_picks = [{"artist": "MB Band", "title": "MB Track"}]
+        play_called = []
+
+        async def fake_play(b, m):
+            play_called.append(m.text)
+
+        music_mod = MagicMock()
+        music_mod.cmd_play = fake_play
+        bot.registry._modules["music"] = music_mod
+
+        with patch.dict("os.environ", {}, clear=True), \
+             patch("plugins._musicbrainz.mb_search_artist",
+                   return_value="mbid-123"), \
+             patch("plugins._musicbrainz.mb_artist_tags",
+                   return_value=["rock"]), \
+             patch("plugins._musicbrainz.mb_find_similar_recordings",
+                   return_value=mb_picks):
+            asyncio.run(_mod.cmd_similar(bot, msg))
+        assert len(play_called) == 1
+        assert "MB Band" in play_called[0]
 
     def test_no_artist_nothing_playing(self):
         bot = _FakeBot()
@@ -468,12 +512,29 @@ class TestCmdSimilar:
 
 
 class TestCmdTags:
-    def test_no_api_key(self):
+    def test_no_api_key_mb_fallback(self):
+        """No API key falls back to MusicBrainz for tags."""
         bot = _FakeBot(api_key="")
         msg = _Msg(text="!tags Tool")
-        with patch.dict("os.environ", {}, clear=True):
+        with patch.dict("os.environ", {}, clear=True), \
+             patch("plugins._musicbrainz.mb_search_artist",
+                   return_value="mbid-123"), \
+             patch("plugins._musicbrainz.mb_artist_tags",
+                   return_value=["rock", "progressive metal", "art rock"]):
             asyncio.run(_mod.cmd_tags(bot, msg))
-        assert any("not configured" in r for r in bot.replied)
+        assert any("Tool:" in r for r in bot.replied)
+        assert any("rock" in r for r in bot.replied)
+        assert any("progressive metal" in r for r in bot.replied)
+
+    def test_no_api_key_mb_no_results(self):
+        """No API key + MusicBrainz returns nothing shows 'no tags'."""
+        bot = _FakeBot(api_key="")
+        msg = _Msg(text="!tags Obscure")
+        with patch.dict("os.environ", {}, clear=True), \
+             patch("plugins._musicbrainz.mb_search_artist",
+                   return_value=None):
+            asyncio.run(_mod.cmd_tags(bot, msg))
+        assert any("No tags found" in r for r in bot.replied)
 
     def test_no_artist_nothing_playing(self):
         bot = _FakeBot()