feat: add YouTube search to !play and fix NA URL fallback

Non-URL input (e.g. !play classical music) searches YouTube for 10
results and picks one randomly. Also fixes --flat-playlist returning
"NA" as the URL for single videos by falling back to the original
input URL.

tests/test_music.py

@@ -134,6 +134,25 @@ class TestPlayCommand:
         assert len(ps["queue"]) == 1
         assert ps["queue"][0].title == "Test Track"
 
+    def test_play_search_query(self):
+        bot = _FakeBot()
+        msg = _Msg(text="!play classical music")
+        tracks = [
+            ("https://youtube.com/watch?v=a", "Result 1"),
+            ("https://youtube.com/watch?v=b", "Result 2"),
+            ("https://youtube.com/watch?v=c", "Result 3"),
+        ]
+        with patch.object(_mod, "_resolve_tracks", return_value=tracks) as mock_rt:
+            with patch.object(_mod, "_ensure_loop"):
+                asyncio.run(_mod.cmd_play(bot, msg))
+        # Should prepend ytsearch10: for non-URL input
+        mock_rt.assert_called_once()
+        assert mock_rt.call_args[0][0] == "ytsearch10:classical music"
+        # Should pick one random result, not enqueue all
+        ps = _mod._ps(bot)
+        assert len(ps["queue"]) == 1
+        assert any("Playing" in r for r in bot.replied)
+
     def test_play_shows_queued_when_busy(self):
         bot = _FakeBot()
         ps = _mod._ps(bot)
@@ -354,6 +373,21 @@ class TestMusicHelpers:
         assert len(result) == 80
         assert result.endswith("...")
 
+    def test_is_url_http(self):
+        assert _mod._is_url("https://youtube.com/watch?v=abc") is True
+
+    def test_is_url_plain_http(self):
+        assert _mod._is_url("http://example.com") is True
+
+    def test_is_url_ytsearch(self):
+        assert _mod._is_url("ytsearch:classical music") is True
+
+    def test_is_url_search_query(self):
+        assert _mod._is_url("classical music") is False
+
+    def test_is_url_single_word(self):
+        assert _mod._is_url("jazz") is False
+
 
 # ---------------------------------------------------------------------------
 # TestPlaylistExpansion
@@ -431,6 +465,14 @@ class TestPlaylistExpansion:
             tracks = _mod._resolve_tracks("https://example.com/v1")
         assert tracks == [("https://example.com/v1", "Single Video")]
 
+    def test_resolve_tracks_na_url_fallback(self):
+        """--flat-playlist prints NA for single videos; use original URL."""
+        result = MagicMock()
+        result.stdout = "NA\nSingle Video\n"
+        with patch("subprocess.run", return_value=result):
+            tracks = _mod._resolve_tracks("https://example.com/v1")
+        assert tracks == [("https://example.com/v1", "Single Video")]
+
     def test_resolve_tracks_playlist(self):
         """Subprocess returning multiple url+title pairs."""
         result = MagicMock()