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/internal/c2_client.py

@@ -68,6 +68,28 @@ class C2Client:
             logger.error(f"C2 stop_guild failed: {e}")
             return None
 
+    async def redeem_guild_link(self, guild_id: int | str, code: str, discord_user_id: str,
+                                guild_name: Optional[str]) -> tuple[bool, dict]:
+        """/drb link code: (server-26#213). Returns (ok, body); a refusal body
+        carries C2's own ``detail`` text, which is written for the end user."""
+        try:
+            async with httpx.AsyncClient(timeout=10) as client:
+                r = await client.post(
+                    f"{self.base}/discord/guilds/{guild_id}/link/redeem",
+                    json={"code": code, "discord_user_id": discord_user_id, "guild_name": guild_name},
+                    headers=self._headers(),
+                )
+                body = r.json() if r.headers.get("content-type", "").startswith("application/json") else {}
+                if r.status_code == 429:
+                    return False, {"detail": "Too many link attempts from this server. Wait ten minutes "
+                                             "and try again.", "code": "rate_limited"}
+                if r.status_code >= 400:
+                    return False, body if isinstance(body, dict) else {"detail": str(body)}
+                return True, body
+        except Exception as e:
+            logger.error(f"C2 redeem_guild_link failed: {e}")
+            return False, {"detail": "Could not reach DRB. Try again in a minute.", "code": "c2_unreachable"}
+
     # ------------------------------------------------------------------
     # Trips
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