fix: complete autoscale scale-in reliably and reject new INVITEs while draining (#248)

Scale-in completion never happened: app.js holds the placeholder
Emitter that autoscale-manager returns synchronously (the real
SnsNotifier replaces it later inside an async IIFE), so the completion
poller never saw operationalState change; it also called the
nonexistent scaleIn() rather than completeScaleIn().  Instances in
Terminating:Wait therefore always burned the full lifecycle hook
heartbeat timeout.

In addition, nothing consumed dryUpCalls: a draining SBC kept accepting
new INVITEs sent directly to its public address right up until
termination.

Changes:
- complete the scale-in from within the ScaleIn handler in
  autoscale-manager, where the real notifier is in scope
- while draining, reject new INVITEs with 503 so senders fail over to
  another SBC (INVITE with Replaces is allowed through since it targets
  a call already in progress here)
- a server may run several sbc-inbound and sbc-outbound processes, and
  completing the hook when only this process is idle would terminate
  the instance while sibling processes still have calls; each process
  now reports its call count to redis (lib/call-count-reporter.js, with
  a companion change in sbc-outbound) and the draining process
  completes only when the server-wide count is zero on two consecutive
  checks, falling back to its own count if no reports are present

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

app.js

@@ -155,6 +155,17 @@ srf.locals = {
 };
 const activeCallIds = srf.locals.activeCallIds;
 
+/* report our call count to redis so a draining process can count calls across
+   all sbc-inbound and sbc-outbound processes on this server */
+if (!process.env.K8S && 'test' !== process.env.NODE_ENV) {
+  srf.locals.callCountReporter = require('./lib/call-count-reporter')({
+    logger,
+    addKey,
+    addToSet,
+    getCount: () => activeCallIds.size
+  });
+}
+
 const {
   initLocals,
   handleSipRec,