Discord: several radio streams per voice channel; adding never overwrites (#213)

Owner-reported on live: adding a radio channel for a server + voice channel
that already had one (different label, system, even bot) overwrote the
existing stream and made its bot leave, because stream_id was guild:channel.

- New streams get guild:channel:<12 hex> ids (leases.new_stream_id); old
  guild:channel docs keep working for read/stop/remove/node reports. Route
  validators accept both (leases.STREAM_ID_PATTERN).
- start_stream: no stream_id = ADD. Same system already on in that channel
  is idempotent (already_on, label/mode untouched); a stopped one of that
  system is restarted; anything else is a new doc. stream_id = explicit
  restart/edit (admin Start button, reconciler), incl. moving to another bot.
- preferred_token_id is strict: bot already in this server -> 409
  preferred_busy naming the stream; on another node -> preferred_other_node;
  unusable -> preferred_unavailable. No silent swap. pool_exhausted now says
  one-bot-per-server and to add + invite another bot.
- One-token-per-guild was already enforced inside the lease transaction;
  now covered by race tests. Per-guild asyncio lock serialises the
  already-on / server-cap check with the create (guild half of #223).
- Stop by stream_id (POST /discord/guilds/{id}/stop {stream_id}); admin UI
  Stop/Start/Remove act on the row's stream; node picker lists every stream
  in a channel and adds beside other systems.
- Bot: /leave stream: (autocomplete) stops one; /leave channel: with several
  streams asks which unless all:True; bare /leave still stops the server.
  /status and /join name each stream's bot.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

drb-server-discord-bot/app/internal/c2_client.py

@@ -67,12 +67,14 @@ class C2Client:
             return None
 
     async def stop_guild(self, guild_id: int | str, channel_id: Optional[str] = None,
-                         reason: str = "leave") -> Optional[list]:
+                         reason: str = "leave", stream_id: Optional[str] = None) -> Optional[list]:
+        """Stop radio in one server: everything, one voice channel (every
+        stream in it), or one stream by id. C2 never reaches past the guild."""
         try:
             async with httpx.AsyncClient(timeout=30) as client:
                 r = await client.post(
                     f"{self.base}/discord/guilds/{guild_id}/stop",
-                    json={"channel_id": channel_id, "reason": reason},
+                    json={"channel_id": channel_id, "stream_id": stream_id, "reason": reason},
                     headers=self._headers(),
                 )
                 r.raise_for_status()