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/components/NodeDiscordRadio.tsx

@@ -5,6 +5,10 @@
 // the Discord servers linked to this node's org and the voice channels the
 // DRB bot reported in them; a pick becomes a radio channel pinned to this node
 // (desired state — the node reconciles and reports back, shown per row).
+// A voice channel can carry several radio streams, each its own bot playing
+// its own system (server-26#213 follow-up): picking a channel that already
+// plays another system ADDS this node's system beside it; only a stream of
+// this node's own system there is restarted / moved here.
 // Layout follows design-canvas/discord-system-mockup.html "Radio channels".
 
 import { useCallback, useEffect, useMemo, useState } from "react";
@@ -64,6 +68,11 @@ function modeLabel(mode: DiscordStreamView["mode"]) {
   return mode === "persistent" ? "persistent" : "ad-hoc";
 }
 
+function streamLabel(s: DiscordStreamView) {
+  const sys = s.system_name ?? "radio";
+  return s.token_name ? `${sys} (${s.token_name})` : sys;
+}
+
 // ---------------------------------------------------------------------------
 // Picker
 // ---------------------------------------------------------------------------
@@ -199,6 +208,7 @@ function DiscordPicker({
                       <div className="space-y-1">
                         {grp.channels.map((c) => {
                           const here = c.stream?.node_id === nodeId;
+                          const others = (c.streams ?? []).filter((x) => x.stream_id !== c.stream?.stream_id);
                           return (
                             <label
                               key={c.channel_id}
@@ -221,8 +231,14 @@ function DiscordPicker({
                                 {c.stream && (
                                   <span className="text-xs text-ink-muted block">
                                     {here
-                                      ? "Already on this node"
-                                      : `Carried by ${c.stream.node_name ?? "another node"} — picking it moves it to this node`}
+                                      ? `${options.system_name ?? "This system"} is already on this node here`
+                                      : `${options.system_name ?? "This system"} is carried by ${c.stream.node_name ?? "another node"} — picking it moves it to this node`}
+                                  </span>
+                                )}
+                                {others.length > 0 && (
+                                  <span className="text-xs text-ink-muted block">
+                                    Also playing: {others.map(streamLabel).join(", ")}
+                                    {!c.stream && ` — this adds ${options.system_name ?? "this node's system"} beside it with another bot`}
                                   </span>
                                 )}
                               </span>
@@ -356,7 +372,7 @@ export function NodeDiscordRadio({ nodeId }: { nodeId: string }) {
 
   async function remove(s: DiscordStreamView) {
     const name = s.channel_name ?? "this channel";
-    if (!confirm(`Remove radio from ${name} permanently? It is stopped first (the node logs its radio bot out), then deleted from DRB.`)) return;
+    if (!confirm(`Remove ${streamLabel(s)} from ${name} permanently? It is stopped first (the node logs its radio bot out), then deleted from DRB. Other streams in that voice channel keep playing.`)) return;
     setBusy(`rm:${s.stream_id}`);
     setActionError(null);
     try {
@@ -429,7 +445,9 @@ export function NodeDiscordRadio({ nodeId }: { nodeId: string }) {
                 <tr key={s.stream_id} className="border-b border-line last:border-0 align-top">
                   <td className="px-4 py-2.5">
                     <div className="text-ink font-medium">🔊 {s.channel_name ?? s.channel_id}</div>
-                    <div className="text-xs text-ink-muted">{s.guild_name ?? s.guild_id}</div>
+                    <div className="text-xs text-ink-muted">
+                      {s.guild_name ?? s.guild_id} · {streamLabel(s)}
+                    </div>
                   </td>
                   <td className="px-4 py-2.5 text-ink-2">{modeLabel(s.mode)}</td>
                   <td className="px-4 py-2.5">