feat: automatic proxy re-sync on settings change

When domain, SSL certificate, or proxy provider changes in settings:
- Delete old proxy routes from the previous provider
- Switch to None: clear all route IDs on instances
- Switch to NPM/Traefik: re-create routes with new settings
- Domain change: re-configure all routes with new FQDN
- SSL cert change: re-apply to all existing routes
- Provider created dynamically at runtime via createProxyProvider()
- Deployer and API server updated via SetProxyProvider callback

cmd/server/main.go

@@ -290,6 +290,9 @@ func main() {
 	apiServer.SetDNSProviderChangedCallback(func(provider dns.Provider) {
 		dep.SetDNSProvider(provider)
 	})
+	apiServer.SetProxyProviderChangedCallback(func(provider proxy.Provider) {
+		dep.SetProxyProvider(provider)
+	})
 	router := apiServer.Router()
 
 	// Serve embedded static files for the SPA frontend.