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/main.py

@@ -20,6 +20,7 @@ from app.internal.auth import (
 from app.routers import nodes, systems, calls, upload, tokens, incidents, alerts, admin, trips, places, links, users
 from app.routers import enrollment, media, org, waitlist, telemetry, replay, traffic, cameras
 from app.routers import discord as discord_router
+from app.routers import discord_link as discord_link_router
 from app.internal import dynsec
 from app.internal import firestore as fstore
 
@@ -158,6 +159,7 @@ app.include_router(media.router)
 # auth is per-endpoint: bot routes take the service key (or a platform admin);
 # the token-fetch route takes the calling node's own API key.
 app.include_router(discord_router.router)
+app.include_router(discord_link_router.router)  # auth per-endpoint (org owner / bot service key) — server-26#213
 # NOTE: there used to be an app.routers.mqtt_auth router here (an HTTP
 # backend for the mosquitto-go-auth plugin). That plugin's upstream project
 # is archived (no CVE patches) and was rejected for an internet-facing