Add certificate issuance, CSR signing, and cert listing to web UI

- Add SignCSR RPC to v2 CA proto and regenerate; implement handleSignCSR
  in CA engine and caServer gRPC layer; add SignCSR client method and
  POST /pki/sign-csr web route with result display in pki.html
- Fix issuer detail cert listing: template was using map-style index on
  CertSummary structs; switch to struct field access and populate
  IssuedBy/IssuedAt fields from proto response
- Add certificate detail view (cert_detail.html) with GET /cert/{serial}
  and GET /cert/{serial}/download routes
- Update Makefile proto target to generate both v1 and v2 protos

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

web/templates/issuer_detail.html

@@ -45,7 +45,7 @@
             <tbody>
             {{range .Certs}}
             <tr>
-                <td>{{.CommonName}}</td>
+                <td><a href="/pki/cert/{{.Serial}}">{{.CommonName}}</a></td>
                 <td>{{.Profile}}</td>
                 <td><code>{{.Serial}}</code></td>
                 <td>{{.IssuedBy}}</td>