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>

infra/step-ca/templates/federation.tpl

@@ -5,41 +5,49 @@
   {{- /*
     Mosaic Federation X.509 Certificate Template
     ============================================
-    This template is used by the "mosaic-fed" JWK provisioner to sign
-    federation client certificates.
+    Provisioner: mosaic-fed (JWK)
+    Implemented: FED-M2-04
 
-    Custom OID extensions (per PRD §6):
-      1.3.6.1.4.1.99999.1  — mosaic.federation.grantId   (UUID string)
-      1.3.6.1.4.1.99999.2  — mosaic.federation.subjectUserId (UUID string)
+    This template emits two custom OID extensions carrying Mosaic federation
+    identifiers. The OTT token (built by CaService.buildOtt) MUST include the
+    claims `mosaic_grant_id` and `mosaic_subject_user_id` as top-level JWT
+    claims. step-ca exposes them under `.Token.<claim>` in this template.
 
-    TODO (M2-04): Wire actual OID extensions below once the CA service
-    (apps/gateway/src/federation/ca.service.ts) lands the SAN-bearing CSR
-    work and the template can be exercised end-to-end.
+    OID Registry (Mosaic Internal Arc — 1.3.6.1.4.1.99999):
+      1.3.6.1.4.1.99999.1  mosaic_grant_id        (UUID, 36 ASCII chars)
+      1.3.6.1.4.1.99999.2  mosaic_subject_user_id  (UUID, 36 ASCII chars)
+
+    DER encoding for each extension value (ASN.1 UTF8String):
+      Tag    = 0x0C  (UTF8String)
+      Length = 0x24  (decimal 36 — the fixed length of a UUID string)
+      Value  = 36 ASCII bytes of the UUID
+
+    The `printf` below builds the raw TLV bytes then base64-encodes them.
+    step-ca expects the `value` field to be base64-encoded raw DER bytes.
+
+    Fail-loud contract:
+      If either claim is missing from the token the printf will produce a
+      zero-length UUID field, making the extension malformed. step-ca will
+      reject the certificate rather than issuing one without the required OIDs.
+      Silent OID stripping is NEVER tolerated.
 
     Step-CA template reference:
       https://smallstep.com/docs/step-ca/templates
-
-    Expected final shape of the extensions block (placeholder — not yet
-    activated):
-
-    "extensions": [
-      {
-        "id": "1.3.6.1.4.1.99999.1",
-        "critical": false,
-        "value": {{ toJson (first .Token.mosaic_grant_id) }}
-      },
-      {
-        "id": "1.3.6.1.4.1.99999.2",
-        "critical": false,
-        "value": {{ toJson (first .Token.mosaic_subject_user_id) }}
-      }
-    ],
-
-    The provisioner must pass these values in the ACME/JWK token payload
-    (token claims `mosaic_grant_id` and `mosaic_subject_user_id`) when
-    submitting the CSR.  M2-04 owns that work.
   */ -}}
 
+  "extensions": [
+    {
+      "id": "1.3.6.1.4.1.99999.1",
+      "critical": false,
+      "value": "{{ printf "\x0c\x24%s" .Token.mosaic_grant_id | b64enc }}"
+    },
+    {
+      "id": "1.3.6.1.4.1.99999.2",
+      "critical": false,
+      "value": "{{ printf "\x0c\x24%s" .Token.mosaic_subject_user_id | b64enc }}"
+    }
+  ],
+
   "keyUsage": ["digitalSignature"],
   "extKeyUsage": ["clientAuth"],
   "basicConstraints": {