Add edge routing and health check RPCs (Phase 2)

New agent RPCs for v2 multi-node orchestration:

- SetupEdgeRoute: provisions TLS cert from Metacrypt, resolves backend
  hostname to Tailnet IP, validates it's in 100.64.0.0/10, registers
  L7 route in mc-proxy. Rejects backend_tls=false.
- RemoveEdgeRoute: removes mc-proxy route, cleans up TLS cert, removes
  registry entry.
- ListEdgeRoutes: returns all edge routes with cert serial/expiry.
- HealthCheck: returns agent health and container count.

New database table (migration 4): edge_routes stores hostname, backend
info, and cert paths for persistence across agent restarts.

ProxyRouter gains CertPath/KeyPath helpers for consistent cert path
construction.

Security:
- Backend hostname must resolve to a Tailnet IP (100.64.0.0/10)
- backend_tls=false is rejected (no cleartext to backends)
- Cert provisioning failure fails the setup (no route to missing cert)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/agent/proxy.go

@@ -48,6 +48,16 @@ func (p *ProxyRouter) Close() error {
 	return p.client.Close()
 }
 
+// CertPath returns the expected TLS certificate path for a given name.
+func (p *ProxyRouter) CertPath(name string) string {
+	return filepath.Join(p.certDir, name+".pem")
+}
+
+// KeyPath returns the expected TLS key path for a given name.
+func (p *ProxyRouter) KeyPath(name string) string {
+	return filepath.Join(p.certDir, name+".key")
+}
+
 // GetStatus returns the mc-proxy server status.
 func (p *ProxyRouter) GetStatus(ctx context.Context) (*mcproxy.Status, error) {
 	if p == nil {