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/DiscordServersCard.tsx

@@ -329,12 +329,13 @@ export function DiscordServersCard({ canEdit, isAdmin }: { canEdit: boolean; isA
 
       <div className="px-5 py-3 border-t border-line bg-raised text-xs text-ink-muted">
         {restricted ? (
-          <>Unlinked servers see only <b className="text-ink">/drb link</b> and <b className="text-ink">/help</b>. </>
+          <>Where the DRB server bot is present, an unlinked server sees only <b className="text-ink">/drb link</b> and <b className="text-ink">/help</b>. </>
         ) : isAdmin ? (
           <>Platform setting: unlinked servers still act for the default organization (<span className="font-mono">DISCORD_UNLINKED_GUILD_ORG_ID</span>) until linked. </>
         ) : null}
-        Every command and autocomplete is scoped to the linked organization — no server sees another
-        organization&apos;s nodes, systems or bots.
+        Radio channels, and every DRB server bot command and autocomplete, are scoped to the linked organization — no
+        server sees another organization&apos;s nodes, systems or bots. The DRB server bot is optional: a radio bot
+        alone is enough to stream.
       </div>
     </Card>
   );