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.

docs/CHEATSHEET.md

@@ -548,6 +548,7 @@ HTML stripped on receive, escaped on send. IRC-only commands are no-ops.
 ```
 !play <url|playlist>     # Play audio (YouTube, SoundCloud, etc.)
 !play <playlist-url>     # Playlist tracks expanded into queue
+!play classical music    # YouTube search, random pick from top 10
 !stop                    # Stop playback, clear queue
 !skip                    # Skip current track
 !queue                   # Show queue

docs/USAGE.md

@@ -1566,6 +1566,7 @@ and voice transmission.
 
 ```
 !play <url|playlist> Play audio or add to queue (playlists expanded)
+!play <query>        Search YouTube, play a random result
 !stop                Stop playback, clear queue
 !skip                Skip current track
 !queue               Show queue
@@ -1576,6 +1577,8 @@ and voice transmission.
 ```
 
 - Queue holds up to 50 tracks
+- Non-URL input is treated as a YouTube search; 10 results are fetched
+  and one is picked randomly
 - Playlists are expanded into individual tracks; excess tracks are
   truncated at the queue limit
 - Volume changes ramp smoothly over ~200ms (no abrupt jumps)