feat(federation): mTLS AuthGuard with OID-based grant resolution (FED-M3-03)

Adds FederationAuthGuard that validates inbound mTLS client certs on
federation API routes. Extracts custom OIDs (grantId, subjectUserId),
loads the grant+peer from DB in one query, asserts active status, and
validates cert serial as defense-in-depth. Attaches FederationContext
to requests on success and uses federation wire-format error envelopes
(not raw NestJS exceptions) for 401/403 responses.

New files:
- apps/gateway/src/federation/oid.util.ts — shared OID extraction (no dupe ASN.1 logic)
- apps/gateway/src/federation/server/federation-auth.guard.ts — guard impl
- apps/gateway/src/federation/server/federation-context.ts — FederationContext type + module augment
- apps/gateway/src/federation/server/index.ts — barrel export
- apps/gateway/src/federation/server/__tests__/federation-auth.guard.spec.ts — 11 unit tests

Modified:
- apps/gateway/src/federation/grants.service.ts — adds getGrantWithPeer() with join
- apps/gateway/src/federation/federation.module.ts — registers FederationAuthGuard as provider

Closes #462

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

apps/gateway/src/federation/federation.module.ts

@@ -6,10 +6,24 @@ import { EnrollmentService } from './enrollment.service.js';
 import { FederationController } from './federation.controller.js';
 import { GrantsService } from './grants.service.js';
 import { FederationClientService } from './client/index.js';
+import { FederationAuthGuard } from './server/index.js';
 
 @Module({
   controllers: [EnrollmentController, FederationController],
-  providers: [AdminGuard, CaService, EnrollmentService, GrantsService, FederationClientService],
-  exports: [CaService, EnrollmentService, GrantsService, FederationClientService],
+  providers: [
+    AdminGuard,
+    CaService,
+    EnrollmentService,
+    GrantsService,
+    FederationClientService,
+    FederationAuthGuard,
+  ],
+  exports: [
+    CaService,
+    EnrollmentService,
+    GrantsService,
+    FederationClientService,
+    FederationAuthGuard,
+  ],
 })
 export class FederationModule {}