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/duration.ts

@@ -0,0 +1,17 @@
+const DURATION_PATTERN = /^(\d+)(ms|s|m|h)$/;
+
+const UNIT_MS: Record<string, number> = {
+  ms: 1,
+  s: 1_000,
+  m: 60_000,
+  h: 3_600_000,
+};
+
+export function parseDuration(value: string): number {
+  const match = DURATION_PATTERN.exec(value.trim());
+  if (!match) {
+    throw new Error(`Invalid duration "${value}" (expected e.g. "500ms", "5s", "1m", "1h")`);
+  }
+  const [, amount, unit] = match;
+  return Number(amount) * UNIT_MS[unit];
+}