Refactor nginx to path-based domain model with config migration

Replace the legacy top-level management key with a unified paths-based
model. Each domain now contains a paths map where each entry defines its
own backend, auth, headers, and flags (is_management, is_websocket).

- Add _migrate_config() to auto-migrate legacy formats on first load
- Remove set_management_proxy() and POST_NGINX_MANAGEMENT endpoint
- Update server_block.conf template to iterate paths with per-location auth
- Update daemon handler, API blueprint, state collector, and install script
- Add server config generation tests for paths, WebSocket, auth inheritance
- Update frontend proxy page to display per-path rows with flags

daemon/iface.py

@@ -43,7 +43,7 @@ POST_NGINX_DOMAINS_UPDATE: Endpoint = _ep("POST", "/nginx/domains/update")
 POST_NGINX_APPLY: Endpoint = _ep("POST", "/nginx/apply")
 POST_NGINX_TEST: Endpoint = _ep("POST", "/nginx/test")
 POST_NGINX_SSL_APPLY: Endpoint = _ep("POST", "/nginx/ssl-apply")
-POST_NGINX_MANAGEMENT: Endpoint = _ep("POST", "/nginx/management")
+
 POST_NGINX_RELOAD: Endpoint = _ep("POST", "/nginx/reload")
 
 # ---- Firewall ----