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-frontend/app/settings/organization/page.tsx

@@ -10,6 +10,7 @@ import { Card, CardHeader } from "@/components/ui/Card";
 import { Badge } from "@/components/ui/Badge";
 import { Button } from "@/components/ui/Button";
 import { Skeleton } from "@/components/ui/Skeleton";
+import { DiscordServersCard } from "@/components/DiscordServersCard";
 
 function StatTile({ label, value }: { label: string; value: string | number }) {
   return (
@@ -118,6 +119,8 @@ export default function OrganizationSettingsPage() {
         </div>
       </Card>
 
+      <DiscordServersCard canEdit={canEdit} isAdmin={isAdmin} />
+
       <div className="bg-gray-900 border border-red-800/60 rounded-xl p-5">
         <CardHeader title="Danger zone" subtitle="Destructive organization-level actions." />
         <div className="flex flex-wrap gap-3">