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/discordPicker.ts

@@ -18,7 +18,10 @@ export interface DiscordStreamView {
   channel_id: string;
   guild_name: string | null;
   channel_name: string | null;
+  system_id?: string | null;
   system_name: string | null;
+  /** The radio bot carrying it — tells apart several streams in one voice channel. */
+  token_name?: string | null;
   mode: "persistent" | "adhoc" | null;
   node_id: string | null;
   node_name: string | null;
@@ -35,7 +38,10 @@ export interface DiscordVoiceChannel {
   channel_id: string;
   name: string;
   category: string | null;
+  /** This node's system's stream in the channel, if any — what a pick restarts or moves. */
   stream: DiscordStreamView | null;
+  /** Every live stream in the channel (several systems can share it, each its own bot). */
+  streams?: DiscordStreamView[];
 }
 
 export interface DiscordGuildOption {