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-server-discord-bot/app/commands/radio.py

@@ -284,7 +284,9 @@ class RadioCommands(commands.Cog):
             value=(
                 "Link this server to your DRB organization — needed before anything below works. "
                 "An organization owner gets the code in DRB under Settings → Organization → Discord servers; "
-                "someone with Manage Server runs this."
+                "someone with Manage Server runs this.\n"
+                "Or skip this bot entirely: **Connect a Discord server** on that same page adds a radio bot "
+                "and links the server in one step, and radio channels can be started from a node's page in DRB."
             ),
             inline=False,
         )
@@ -314,7 +316,9 @@ class RadioCommands(commands.Cog):
             name="Radio bots",
             value=(
                 "A radio bot has to be a member of this server before it can join a channel — "
-                "if none is, `/join` replies with an invite link.\n"
+                "if none is, `/join` replies with an invite link. It needs View Channel, Connect and Speak "
+                "there; if it can't, `/join` says which permission is missing and where to allow it.\n"
+                "Radio bots work without this bot: it only adds these commands.\n"
                 "The bot's green speaking ring lights up only while radio is transmitting."
             ),
             inline=False,