S7 review: align OAuth UI with S6's actual contract (server-26#225)

- complete: S6 returns streams_stopped, not stopped — the "N radio
  channels were stopped" note never showed.
- start: bot name is nested as bot.name.
- "The bot for a system" with an org-owned preferred bot now sends
  token_id (S6 accepts it today); system_id only for a platform-pinned
  pool bot, which still needs S6 to accept system_id.
- A 404 for one chosen bot no longer hides Connect and the picker
  (the message told the user to try Automatic, but the button was gone).
- Server count uses GET /tokens' per-bot server_cap when present.

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

drb-frontend/lib/discord.ts

@@ -30,6 +30,8 @@ export interface RadioBot {
   /** "Connect a Discord server" is on for this bot: its OAuth client secret is
    *  set server-side (server-26#225 S6). The secret itself is never sent. */
   oauth_enabled?: boolean | null;
+  /** GET /tokens (S6): the pool cap for a DRB pool bot, null for a BYO bot. */
+  server_cap?: number | null;
 }
 
 export interface RadioBotAddResult {