feat(federation): Step-CA client service for grant certs (FED-M2-04)

- Add CaService (@Injectable) that POSTs CSRs to step-ca /1.0/sign over
  HTTPS with a pinned CA root cert; builds HS256 OTT with custom claims
  mosaic_grant_id and mosaic_subject_user_id plus step.sha CSR fingerprint
- Add CaServiceError with cause + remediation for fail-loud contract
- Add IssueCertRequestDto and IssuedCertDto with class-validator decorators
- Add FederationModule exporting CaService; wire into AppModule
- Replace federation.tpl TODO placeholder with real step-ca Go template
  emitting OID 1.3.6.1.4.1.99999.1 (grantId) and .2 (subjectUserId) as
  DER UTF8String extensions (tag 0x0C, length 0x24, base64-encoded value)
- Update infra/step-ca/init.sh to patch mosaic-fed provisioner config with
  templateFile path via jq on first boot (idempotent)
- Append OID assignment registry and CA env var table to docs/federation/SETUP.md
- 11 unit tests pass: happy path, certChain fallbacks, HTTP 401/4xx, malformed
  CSR (no HTTP call), non-JSON response, connection error, JWT claim assertions

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

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

@@ -0,0 +1,8 @@
+import { Module } from '@nestjs/common';
+import { CaService } from './ca.service.js';
+
+@Module({
+  providers: [CaService],
+  exports: [CaService],
+})
+export class FederationModule {}