feat(monitor): store nginx telemetry for generic clients

- accept nginx installation status and sites-enabled list
- persist nginx fields in server state
- expose nginx data in monitor overview/admin views
- auto-migrate new server_states columns on startup

app/services/monitoring.py

@@ -304,6 +304,8 @@ def get_server_states_view(db: Session, offline_after_minutes: int = 7):
             'disk_pct': st.disk_pct if st else None,
             'swap_pct': st.swap_pct if st else None,
             'agents': json.loads(st.agents_json) if st and st.agents_json else [],
+            'nginx_installed': st.nginx_installed if st else None,
+            'nginx_sites': json.loads(st.nginx_sites_json) if st and st.nginx_sites_json else [],
             'last_seen_at': last_seen.isoformat() if last_seen else None,
         })
     return out