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/types/widget-result.ts

@@ -0,0 +1,16 @@
+export interface DockerContainerResult {
+  status: "running" | "exited" | "restarting" | "paused" | "dead" | "created" | "unknown";
+  health: "healthy" | "unhealthy" | "starting" | "none";
+  startedAt: string | null;
+  uptimeSeconds: number | null;
+  restartCount: number;
+  cpuPercent: number | null;
+  memUsageBytes: number | null;
+  memLimitBytes: number | null;
+  image: string;
+}
+
+export type WidgetResult =
+  | { type: "docker"; data: DockerContainerResult }
+  | { type: "database"; data: DockerContainerResult }
+  | { type: "error"; message: string };