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-c2-core/app/internal/discord_streams.py

@@ -441,7 +441,9 @@ ERROR_TEXT = {
     "pool_exhausted": ("Every radio bot that can join this server is already busy — a bot can be in only one voice "
                        "channel per server, and a bot carrying radio on another node can't be shared. Add another "
                        "radio bot to DRB and invite it to this server, or stop a radio channel here."),
-    "limit": "This server already has its maximum number of radio channels. Stop one with /leave first.",
+    # Reaches the web picker and /join alike: name both ways to stop one.
+    "limit": "This server already has its maximum number of radio channels. Stop one first (Stop on a node's "
+             "Discord radio panel, or /leave in Discord).",
     "preferred_unavailable": "That radio bot can't be used here (it was removed, failed validation, or belongs to "
                              "another organization). Pick another bot, or let DRB choose any free one.",
     "pool_full": ("No radio bot is in this server, and every DRB radio bot that could be invited is already in its "