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-frontend/lib/discord.ts

@@ -67,6 +67,8 @@ export interface DiscordStream {
   stop_reason?: string | null;
   updated_at?: string | null;
   created_at?: string | null;
+  /** POST /discord/streams: this system was already on in that channel — nothing was replaced. */
+  already_on?: boolean;
 }
 
 export interface DiscordGuild {