feat: M15b — OCSP responder, DER CRL, short-lived exemption, revocation GUI

Backend:
- Embedded OCSP responder: GET /api/v1/ocsp/{issuer_id}/{serial} returns
  signed OCSP responses (good/revoked/unknown) using CA key
- DER-encoded X.509 CRL: GET /api/v1/crl/{issuer_id} returns proper DER CRL
  signed by issuing CA with 24h validity window
- Short-lived cert exemption: certs with profile TTL < 1 hour skip CRL/OCSP
  (expiry is sufficient revocation for ephemeral workloads)
- Extended issuer connector interface with GenerateCRL and SignOCSPResponse
- Local CA implements full CRL/OCSP signing; ACME and step-ca return
  appropriate "use native endpoint" errors
- IssuerConnectorAdapter bridges new methods between layers

Frontend:
- Revoke button on certificate detail page with RFC 5280 reason modal
- Revocation banner with reason display and timestamp
- Revocation status indicators in lifecycle section
- "Revoked" filter option in certificates list
- API client: revokeCertificate() function and Certificate type extensions

Tests: ~31 new tests across connector, service, handler, and adapter layers
Docs: milestones renumbered (M13-M14, M16-M18), M15b marked complete

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

internal/connector/issuer/acme/acme.go

@@ -609,3 +609,13 @@ func parseDERChain(derChain [][]byte) (certPEM string, chainPEM string, serial s
 
 	return
 }
+
+// GenerateCRL is not supported by ACME issuers.
+func (c *Connector) GenerateCRL(ctx context.Context, revokedCerts []issuer.RevokedCertEntry) ([]byte, error) {
+	return nil, fmt.Errorf("ACME issuers do not support CRL generation")
+}
+
+// SignOCSPResponse is not supported by ACME issuers.
+func (c *Connector) SignOCSPResponse(ctx context.Context, req issuer.OCSPSignRequest) ([]byte, error) {
+	return nil, fmt.Errorf("ACME issuers do not support OCSP response signing")
+}