feat(brainstorm-server): 4h configurable idle timeout; close WS on shutdown

The companion shut down after only 30 minutes idle — too short for real
brainstorming, where a single question can sit far longer. And shutdown() never
closed upgraded WebSocket sockets, so an open browser connection could keep the
Node process alive after it was supposed to exit.

- Default idle timeout raised to 4 hours, configurable via BRAINSTORM_IDLE_TIMEOUT_MS
  and start-server.sh --idle-timeout-minutes (validated positive integer).
- Reported as idle_timeout_ms in the server-started JSON / server-info.
- shutdown() now destroys all client sockets so the process exits even with an
  open WebSocket.
- Watchdog check interval is configurable (BRAINSTORM_LIFECYCLE_CHECK_MS, default
  60s) so the lifecycle can be tested without minute-long waits.

Adds lifecycle.test.js (configured timeout reported; idle shutdown exits despite
an open WS — teeth-verified; the start-server flag). Wires ws-protocol,
lifecycle, and stop-server suites into npm test.

Closes #1237
Refs #1689

tests/brainstorm-server/package.json

@@ -2,7 +2,7 @@
   "name": "brainstorm-server-tests",
   "version": "1.0.0",
   "scripts": {
-    "test": "node server.test.js"
+    "test": "node ws-protocol.test.js && node server.test.js && node lifecycle.test.js && bash stop-server.test.sh"
   },
   "dependencies": {
     "ws": "^8.19.0"