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-c2-core/app/config.py

@@ -249,7 +249,11 @@ class Settings(BaseSettings):
     discord_lease_offline_release_seconds: int = 600
     # Org a Discord server that was never linked to an org (discord_guilds/{id})
     # acts for. Default keeps today's single-org deployment working unchanged;
-    # set it to "" to require every server to be linked first.
+    # set it to "" to require every server to be linked first. Do that once
+    # every server that should keep working has been linked (/drb link, or
+    # PUT /discord/guilds/{id}/link) — unlinked servers then get only
+    # /drb link and /help (server-26#213). A server an org explicitly unlinked
+    # never falls back to this org either way.
     discord_unlinked_guild_org_id: str = "founding"
 
     class Config: