S7: OAuth start/options accept system_id; Discord picker QA fixes (server-26#225, #256 #257 #258)

- discord_oauth: POST /discord/oauth/start and GET /options take an optional
  system_id. The system must be the org's own or shared (another org's or
  unknown = 404). Its preferred bot is strict: one that can't connect is a
  404, never a quiet swap to Automatic. No preference = the usual order.
  The picker's "bot for a system" now always sends system_id.
- #257: _channel_state gives a bot whose token Discord rejected its own
  "invalid" state; the picker shows "Radio bot needs replacing" with no
  invite links, instead of "No radio bot".
- #256: Start no longer stays disabled on "Needs fix". It re-checks with
  Discord first (refresh=1) and starts only if the fresh answer allows it.
  "Not a voice channel" still disables Start.
- #258: own tooltip for "Not a voice channel"; "platform admin" -> "DRB
  support"; PreferredBotsCard says plainly that adding your own bot isn't
  self-serve (no support address exists yet, see #47), and a load failure
  gets a plain message + Try again instead of the raw C2 error.

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

drb-frontend/components/discord/VoiceCheckBits.tsx

@@ -29,6 +29,11 @@ const BADGE: Record<ChannelVerdict["kind"], { label: string; cls: string; title?
     title: "A bot could stream here, but each one that can is already in another channel of this server or on another node.",
   },
   no_bot: { label: "No radio bot", cls: "border-sev-moderate/50 text-sev-moderate", title: "No radio bot is in this Discord server yet." },
+  invalid: {
+    label: "Radio bot needs replacing",
+    cls: "border-sev-major/50 text-sev-major",
+    title: "A radio bot is in this server, but Discord rejected its token. Inviting it again won't help — DRB support has to replace it.",
+  },
   unknown: { label: "Couldn't check", cls: "border-line text-ink-muted", title: "Discord didn't answer the check — DRB will still try." },
 };
 
@@ -59,11 +64,19 @@ export function VerdictBadge({ verdict, loading }: { verdict: ChannelVerdict | n
   );
 }
 
-/** The sentence under a "Needs fix" / "Not a voice channel" channel. */
+/** The sentence under a "Needs fix" / "Not a voice channel" / "Radio bot needs replacing" channel. */
 export function FixText({ verdict, channelName }: { verdict: ChannelVerdict; channelName: string | null }) {
   if (verdict.kind === "not_voice") {
     return <span className="block text-xs text-sev-major mt-1">That is not a voice channel. Radio can only play in a voice channel.</span>;
   }
+  if (verdict.kind === "invalid") {
+    return (
+      <span className="block text-xs text-sev-major mt-1">
+        Discord rejected the radio bot in this server, so it can&apos;t play radio. Inviting it again won&apos;t help —
+        DRB support has to replace it.
+      </span>
+    );
+  }
   if (verdict.kind !== "fix") return null;
   return (
     <span className="block text-xs text-sev-major mt-1">