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/lib/discord.ts

@@ -85,6 +85,14 @@ export interface DiscordGuild {
   acting_org_id: string | null;
   acting_org_name: string | null;
   live_streams: number;
+  /** How it was linked: "admin" | "code" | "oauth" (absent on older links). */
+  linked_via?: string | null;
+  /** The DRB server bot (optional, server-26#225): true/false as it last reported, null = never heard. */
+  main_bot_present?: boolean | null;
+  /** Who listed its voice channels: "main_bot" | "radio_bot". */
+  directory_source?: string | null;
+  /** Radio bots Discord last said are in this server (names). */
+  radio_bots?: string[];
 }
 
 export interface DiscordAdminOverview {