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/internal/discord_streams.py

@@ -75,10 +75,16 @@ def _parse_ts(value) -> Optional[datetime]:
 
 async def resolve_guild_org(guild_id: str) -> Optional[str]:
     """The org a Discord server acts for: its ``discord_guilds`` link, else
-    ``settings.discord_unlinked_guild_org_id`` (None when that is empty)."""
+    ``settings.discord_unlinked_guild_org_id`` (None when that is empty).
+
+    A server an org explicitly unlinked (``unlinked_at`` set, server-26#213)
+    gets no fallback: otherwise unlinking org B's server would quietly hand it
+    to the founding org."""
     link = await fstore.doc_get(GUILDS, str(guild_id))
     if link and link.get("org_id"):
         return link["org_id"]
+    if link and link.get("unlinked_at"):
+        return None
     return settings.discord_unlinked_guild_org_id or None