feat: add admin panel at /admin for full dashboard configuration

- Backend: ConfigManager write methods (atomic YAML save), admin CRUD
  router for settings (display/odoo/refresh) and nodes, WS broadcast
  on config changes, fix nmap scan blocking event loop with to_thread
- Frontend: admin UI with tab navigation, overview dashboard, node
  CRUD table with modal form, Odoo/display/refresh settings pages,
  typed API wrappers, active views filtering, config_changed WS handler
- Infra: nginx no-cache headers for HTML, cache-forever for hashed assets
- Fixes: WebSocket reconnect loop (ref pattern), rotation index OOB
  when views shrink, mutable node list cache

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/main.py

@@ -85,13 +85,15 @@ app.add_middleware(
     allow_headers=["*"],
 )
 
-from routers import network, tasks, calendar, services, ws
+from routers import network, tasks, calendar, services, ws, admin
 
+admin.init(app_config)
 app.include_router(network.router)
 app.include_router(tasks.router)
 app.include_router(calendar.router)
 app.include_router(services.router)
 app.include_router(ws.router)
+app.include_router(admin.router)
 
 
 @app.get("/api/health")