feat(M51): add SCEP server (RFC 8894) for MDM and network device enrollment

Implements Simple Certificate Enrollment Protocol with single-endpoint
operation-based dispatch (GetCACaps, GetCACert, PKIOperation), PKCS#7
SignedData CSR extraction with fallback for raw/base64 CSR, challenge
password authentication via CSR attributes, and shared internal/pkcs7
package extracted from EST handler to eliminate code duplication.

24 new tests (11 service + 13 handler) plus 5 shared pkcs7 package tests.

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

internal/api/router/router.go

@@ -238,6 +238,15 @@ func (r *Router) RegisterESTHandlers(est handler.ESTHandler) {
 	r.Register("GET /.well-known/est/csrattrs", http.HandlerFunc(est.CSRAttrs))
 }
 
+// RegisterSCEPHandlers sets up SCEP (RFC 8894) routes.
+// SCEP uses a single endpoint with operation-based dispatch via query parameters.
+// Authentication is via challenge password in the CSR, not TLS client certs or API keys.
+func (r *Router) RegisterSCEPHandlers(scep handler.SCEPHandler) {
+	// SCEP uses a single path; the handler dispatches on ?operation= query param
+	r.Register("GET /scep", http.HandlerFunc(scep.HandleSCEP))
+	r.Register("POST /scep", http.HandlerFunc(scep.HandleSCEP))
+}
+
 // GetMux returns the underlying http.ServeMux for direct access if needed.
 func (r *Router) GetMux() *http.ServeMux {
 	return r.mux