Node page Discord picker: org's linked servers + voice channels, not raw IDs (#213 item 3, #109 item 13)

- c2-core routers/discord_web.py:
  PUT  /discord/guilds/{id}/directory (service key): the server bot pushes each
       server's name + voice channels; merged onto discord_guilds, never touches org_id.
  GET  /discord/nodes/{id}/join-options: the node org's servers (same
       resolve rule as the bot), their voice channels with any live stream,
       radio-bot readiness + invite links, and this node's streams. No tokens.
  POST /discord/nodes/{id}/streams: stream pinned to the node; server must be
       the node org's, channel must be in the bot's directory, guild + node caps.
  POST /discord/nodes/{id}/streams/stop: one stream on this node, or all.
  Control = platform admin, org owner, or operator owning the node.
- nodes.py: legacy raw-ID discord_join now refuses a server outside the node's org.
- discord bot: DirectorySync cog pushes on ready (15-min loop), join/leave,
  rename, voice/category channel changes (debounced).
- frontend: NodeDiscordRadio panel + picker replaces DiscordJoinModal and the
  admin-only Join/Leave buttons.

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

drb-c2-core/app/routers/nodes.py

@@ -168,6 +168,11 @@ async def send_command(
     if cmd.action == "discord_join":
         if not cmd.guild_id or not cmd.channel_id:
             raise HTTPException(400, "guild_id and channel_id are required.")
+        # The raw-ID path never checked the server belongs to the node's org
+        # (server-26#213): one org's node could be streamed into another org's
+        # Discord server. The web picker uses /discord/nodes/{id}/streams now.
+        if await discord_streams.resolve_guild_org(cmd.guild_id) != node.get("org_id"):
+            raise HTTPException(404, "That Discord server is not linked to this node's organization.")
         system_id = discord_streams._effective_system(node)
         if not system_id:
             raise HTTPException(409, "This node is not listening to a system yet.")