S8: main DRB bot optional end to end + owner-flow integration tests (server-26#225)

- Copy: picker, admin Servers/Streams/How-it-works, org Discord card, bot
  /help and the server-limit refusal no longer imply the DRB server bot is
  required; a radio bot alone is enough.
- Admin overview: per-server main_bot_present, radio_bots, linked_via,
  directory_source; Servers tab gets a Bots column.
- discord_web docstring: states the reversed "never calls Discord" rule
  (plan finding 5) and the optional server bot.
- Server bot directory cog: re-push (which drops C2's cached permission
  answers) on overwrite changes, role permission changes and role deletes.
- tests/test_discord_owner_flows.py: one-bot org (BYO bot, owner-set
  preferred, OAuth link, picker, start, reconciler) and full setup (server
  bot + 4 radio bots: placement, shared channel, every refusal explained),
  against a modelled Discord over httpx.MockTransport.
- storage.download_audio removed: no caller on main since #233.

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

drb-frontend/components/discord/StreamsPanel.tsx

@@ -348,7 +348,8 @@ export function StreamsPanel({
       )}
       {overview.streams.length === 0 ? (
         <p className="px-4 py-6 text-sm text-ink-muted">
-          No radio channels yet. Start one here, or run <span className="font-mono">/join</span> in a linked Discord server.
+          No radio channels yet. Start one here or from a node&apos;s Discord radio panel, or run{" "}
+          <span className="font-mono">/join</span> where the DRB server bot is in a linked Discord server.
         </p>
       ) : (
         <Table head={["Server · channel", "Source", "Talkgroups", "Mode", "Desired → actual", "Bot", ""]}>