Add server certificate management UI (Settings > Endpoints > Server Certificate)

Operational Feature: Web UI for TLS certificate management

Problem:
- Server certificate (nginx HTTPS) managed manually via certs/ directory
- No visibility into cert expiry from UI
- Regeneration required command-line OpenSSL knowledge
- No warning when cert approaching expiration
- Agents using thumbprint pinning need new thumbprint after renewal

Solution:
- New admin-only settings page: Settings > Endpoints > Server Certificate
- View current certificate details (CN, SANs, expiry, thumbprint)
- Warning banner when cert expires within 30 days
- Generate self-signed certificate (configure CN, SANs, validity)
- Upload CA-signed certificate (PEM format)
- Automatic nginx restart after certificate changes
- Copy-to-clipboard for agent thumbprint pinning

Frontend:
- plugins/endpoints/frontend/views/ServerCertSettings.vue
  - Certificate status display with expiry countdown
  - Self-signed generation form (CN, SANs, days)
  - CA certificate upload (cert + key files)
  - Documentation collapsibles (requirements, pinning, air-gapped)
- plugins/endpoints/frontend/routes.js
  - Route: /settings/endpoints/server-cert (admin-only)

Backend API:
- plugins/endpoints/api/routes.py
  - GET /api/endpoints/settings/server-cert (view cert details)
    - Uses openssl to extract CN, issuer, dates, thumbprint, SANs
    - Calculates days until expiry (warns if < 30)
    - Returns self-signed status
  - POST /api/endpoints/settings/server-cert/generate
    - Generates self-signed cert with custom CN/SANs/validity
    - Backs up old certificate with timestamp
    - Writes new cert/key to certs/ directory
    - Restarts nginx (docker-compose restart nginx)
    - Audit logged: endpoints.server_cert.generated
  - POST /api/endpoints/settings/server-cert/upload
    - Validates PEM format (cert + unencrypted key)
    - Backs up old certificate
    - Writes uploaded files to certs/
    - Restarts nginx automatically
    - Audit logged: endpoints.server_cert.uploaded
  - All endpoints admin-only (@require_role('admin'))

Navigation:
- plugins/endpoints/plugin.py
  - Added menu item: Settings > Endpoints > Server Certificate
  - Icon: shield-check, Position: 29

Security:
- Admin-only access (certificate management is privileged operation)
- Private key validation (rejects encrypted keys)
- Certificate format validation (PEM-encoded)
- Audit logging for all cert operations
- Automatic backup before replacement (timestamped .backup files)

Operational Notes:
- Certificate changes restart nginx (brief HTTPS interruption)
- Agents using ServerCertThumbprint pinning must be updated after renewal
- Self-signed certs valid 1 year by default (configurable)
- Air-gapped sites: cert files in certs/ directory bundled automatically

Related:
- Complements certs/README.md and regeneration scripts (commit fd6240772e)
- Agents validate via Windows roots or thumbprint pinning (CertificateRenewal.cs)
- Client certificate renewal uses same nginx TLS surface (routes.py:3455)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

plugins/endpoints/plugin.py

@@ -341,6 +341,16 @@ class EndpointsPlugin(BasePlugin):
                                'tasks already registered on the endpoint.',
                 'position': 28,
             },
+            {
+                'group': 'Endpoints',
+                'to': '/settings/endpoints/server-cert',
+                'icon': 'shield-check',
+                'title': 'Server Certificate',
+                'description': 'Manage the TLS certificate nginx uses for HTTPS. '
+                               'Agents validate this certificate via pinning or '
+                               'Windows trusted roots.',
+                'position': 29,
+            },
         ]
 
     def get_navigation_items(self) -> List[Dict]: