feat: live docker/database widgets over WebSocket (M2)

Adds a custom server.ts (http server + Next request handler + a
noServer:true WebSocket server on /ws) so the app can push live data
without polling. A CollectorScheduler runs one interval-based job per
docker/database widget instance, backed by dockerode against
/var/run/docker.sock, and reconciles jobs when config.yml changes.

Config hot-reload is now fully wired: chokidar watches config.yml/.env,
re-validates on change, and broadcasts config:update (or a
non-destructive config:error, keeping the last-good config) to every
connected browser. The client subscribes to per-widget topics over a
single shared WebSocket with exponential-backoff reconnect and
last-result caching for instant resubscribe.

Verified end-to-end against real throwaway containers (nginx, redis):
live CPU/mem/uptime/health streamed over the socket, interval changes
picked up without a server restart, and a broken config.yml correctly
falls back to the last-valid config instead of crashing the app.

lib/config/widgets.ts

@@ -0,0 +1,21 @@
+import type { Config, Widget } from "./schema";
+
+export interface WidgetInstance {
+  id: string;
+  groupName: string;
+  widget: Widget;
+}
+
+export function widgetId(groupIndex: number, widgetIndex: number): string {
+  return `${groupIndex}:${widgetIndex}`;
+}
+
+export function flattenWidgets(config: Config): WidgetInstance[] {
+  const instances: WidgetInstance[] = [];
+  config.groups.forEach((group, groupIndex) => {
+    group.widgets.forEach((widget, widgetIndex) => {
+      instances.push({ id: widgetId(groupIndex, widgetIndex), groupName: group.name, widget });
+    });
+  });
+  return instances;
+}