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/server/federation-context.ts

@@ -0,0 +1,39 @@
+/**
+ * FederationContext — attached to inbound federation requests after successful
+ * mTLS + grant validation by FederationAuthGuard.
+ *
+ * Downstream verb controllers access this via `request.federationContext`.
+ */
+
+/**
+ * Augment FastifyRequest so TypeScript knows about the federation context
+ * property that FederationAuthGuard attaches on success.
+ */
+declare module 'fastify' {
+  interface FastifyRequest {
+    federationContext?: FederationContext;
+  }
+}
+
+/**
+ * Typed context object attached to the request by FederationAuthGuard.
+ * Carries all data extracted from the mTLS cert + grant DB row needed
+ * by downstream federation verb handlers.
+ */
+export interface FederationContext {
+  /** The federation grant ID extracted from OID 1.3.6.1.4.1.99999.1 */
+  grantId: string;
+
+  /** The local subject user whose data is accessible under this grant */
+  subjectUserId: string;
+
+  /** The peer gateway ID (from the grant's peerId FK) */
+  peerId: string;
+
+  /**
+   * Grant scope — determines which resources the peer may query.
+   * Typed as Record<string, unknown> because the full scope schema lives in
+   * scope-schema.ts; downstream handlers should narrow via parseFederationScope.
+   */
+  scope: Record<string, unknown>;
+}