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/app/settings/discord/page.tsx

@@ -43,8 +43,8 @@ function Kpi({ k, v, s }: { k: string; v: string; s: string }) {
 
 function HowItWorks() {
   const steps = [
-    ["1 · Desired", "/join in Discord or this page writes a discord_streams doc."],
-    ["2 · Lease", "C2 picks an online node on that system and a radio bot already in that server, in one transaction."],
+    ["1 · Desired", "A node page's picker, this page, or /join (DRB server bot) writes a discord_streams doc."],
+    ["2 · Lease", "C2 picks an online node on that system and a radio bot already in that server that Discord lets View, Connect and Speak in the channel — the system's preferred bot first — in one transaction."],
     ["3 · Reconcile", "A retained MQTT message tells the node its full channel list (token ids, never token strings)."],
     ["4 · Voice", "The node logs in as the leased bot and plays its PulseAudio capture — never Icecast."],
     ["5 · Actual", "The node reports each channel's real state; this page and the Discord reply show it."],
@@ -54,6 +54,12 @@ function HowItWorks() {
       title="How a radio channel stays up"
       note="Node reboot, C2 sweep, Discord voice drop: the node re-reads its desired state and reconnects; persistent channels whose node went away are re-placed on another node. A channel desired-on but not connected for 2 minutes turns red on the Radio channels tab."
     >
+      <p className="px-4 pt-4 text-xs text-ink-2">
+        <b className="text-ink">The DRB server bot is optional.</b> One radio bot in a Discord server is enough: adding it
+        through <i>Connect a Discord server</i> (Settings → Organization) links the server, and C2 reads its voice
+        channels and permissions with that bot. The server bot adds the slash commands (/join, /leave, /status,
+        /drb link) and pushes its own channel list; both ways keep working side by side.
+      </p>
       <div className="grid gap-2 p-4 grid-cols-1 sm:grid-cols-2 lg:grid-cols-5">
         {steps.map(([t, d]) => (
           <div key={t} className="bg-raised border border-line rounded-lg p-3 text-xs text-ink-muted">