feat(federation): outbound mTLS FederationClient (FED-M3-08)

Implements FederationClientService — a NestJS injectable that dials peer
gateways over mTLS (undici Agent with cert+sealed-key from federation_peers),
invokes list/get/capabilities verbs, validates responses via Zod, and surfaces
all failure modes as typed FederationClientError with a coherent error code
taxonomy (PEER_NOT_FOUND, PEER_INACTIVE, PEER_MISCONFIGURED, NETWORK,
FORBIDDEN, HTTP_{status}, INVALID_RESPONSE).

Per-peer Agent instances are cached in a Map for the service lifetime;
flushPeer(peerId) invalidates the cache for M5/M6 cert rotation and
revocation events.

Wired into FederationModule providers + exports so QuerySourceService
(M3-09) can inject it.

13 unit tests covering all required scenarios via undici MockAgent +
real sealClientKey/unsealClientKey round-trip.

Closes #462

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/gateway/package.json

@@ -73,6 +73,7 @@
     "rxjs": "^7.8.0",
     "socket.io": "^4.8.0",
     "uuid": "^11.0.0",
+    "undici": "^7.24.6",
     "zod": "^4.3.6"
   },
   "devDependencies": {