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/PreferredBotsCard.tsx

@@ -33,7 +33,8 @@ export function PreferredBotsCard({ canEdit }: { canEdit: boolean }) {
     } catch (err) {
       const e = parseC2Error(err);
       if (e.status === 403) setHidden(true);
-      else setLoadError(e.message);
+      // A server error's raw body ("C2 API error 500: ...") means nothing to an owner.
+      else setLoadError(e.status !== null && e.status >= 500 ? "DRB couldn't load them just now." : e.message);
     }
   }, []);
 
@@ -67,15 +68,20 @@ export function PreferredBotsCard({ canEdit }: { canEdit: boolean }) {
         subtitle="Which of your radio bots plays each system in Discord. DRB tries it first for every new radio channel of that system; if it can't (busy in that server, missing a permission, or not invited), another free bot is used."
       />
       {loadError ? (
-        <p role="alert" className="text-sm text-sev-major">Couldn&apos;t load radio bots: {loadError}</p>
+        <p role="alert" className="text-sm text-sev-major">
+          Couldn&apos;t load radio bots: {loadError}{" "}
+          <button type="button" onClick={() => load()} className="text-accent hover:underline">
+            Try again
+          </button>
+        </p>
       ) : !data ? (
         <p className="text-sm text-ink-muted">Loading…</p>
       ) : data.systems.length === 0 ? (
         <p className="text-sm text-ink-muted">Your organization has no radio systems yet.</p>
       ) : bots.length === 0 ? (
         <p className="text-sm text-ink-muted">
-          Your organization has no radio bots of its own, so DRB picks a free DRB radio bot for each channel. To use
-          your own bot for a system, ask DRB support to add it to your organization.
+          Your organization has no radio bots of its own, so DRB picks a free DRB radio bot for each channel. Adding
+          your own bot isn&apos;t self-serve yet: DRB support adds it for you, so ask whoever set up your DRB account.
         </p>
       ) : (
         <div className="divide-y divide-line">