Don't take the server down when the module unmounts

Deploying 0.3.0 to AB crashed the ProdDash server on restart:

  TypeError: Cannot read properties of null (reading 'streams')
      at server.js:273  req.on('close', () => state.streams.delete(res))

teardown() nulls `state` and then ends every open stream, which fires each
request's close handler — and the handler reached back through the `state`
it had just discarded. Any re-init with a browser connected would do it, so
saving admin config while a dashboard was open was enough to stop the whole
server, every module with it.

The stream handler now closes over this mount's Set rather than the module's
state: after teardown it deletes from a set nobody reads, which is harmless.
The same hazard existed anywhere a callback could outlive the mount — the
keepalive ping, a frame arriving from the listener mid-teardown, and the
/measure interval — and each is closed over or guarded now.

Reproduced first (open an SSE client, save config, watch it fall over) and
confirmed fixed against the same sequence.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

modules/spl-meter/module.json

@@ -1,7 +1,7 @@
 {
   "id": "spl-meter",
   "name": "SPL Meter",
-  "version": "0.3.0",
+  "version": "0.3.1",
   "proddash": ">=1.5.0",
   "description": "Calibrated sound level, Leq and RTA from an audio input on this machine",
   "permissions": [