feat(discord): self-serve server->org linking with /drb link (#213 item 1)

- c2-core: POST /discord/link-codes (org owner/admin) mints a single-use,
  10-minute code; minting revokes the org's older codes. The bot redeems it
  via POST /discord/guilds/{id}/link/redeem (service key) inside one
  Firestore transaction, so two servers racing for one code cannot both win
  and a server linked to org A refuses org B's code. GET/DELETE
  /discord/linked-guilds lists/unlinks the caller's org's servers; unlink
  stops that server's streams. Rate-limited (mint per user, redeem per
  server and per Discord user); every mint/link/refusal/unlink audited.
- resolve_guild_org: an explicitly unlinked server no longer falls back to
  DISCORD_UNLINKED_GUILD_ORG_ID. Linking away from the fallback org stops
  the streams it had running for that org.
- Discord bot: /drb link code: (Manage Server, enforced at runtime too);
  GatedTree answers every other command in an unlinked server with how to
  link (autocomplete returns nothing). With the default fallback org set,
  nothing is gated — unchanged behaviour.
- Frontend: "Linked Discord servers" card on Settings -> Organization —
  generate code with countdown + copy, polls for the new link, unlink.
- 16 tests: single-use, two-server race, one-server/two-org race, expiry,
  revoked/malformed codes, wrong-org takeover, idempotent relink, rate
  limits, fallback stop, scoped list, unlink without fallback.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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

@@ -226,6 +226,15 @@ class RadioCommands(commands.Cog):
     @app_commands.command(name="help", description="How to use this bot.")
     async def help(self, interaction: discord.Interaction):
         embed = discord.Embed(title="DRB Radio Bot — Help", color=0x5865f2)
+        embed.add_field(
+            name="/drb link `code`",
+            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."
+            ),
+            inline=False,
+        )
         embed.add_field(
             name="/join `system` [`bot`] [`persistent`]",
             value=(