voicemail: mailbox config, voicemail on no-answer/busy, *97/*98, MWI

Generate voicemail_selfphoneng.conf (03 §7) with one mailbox per extension
that has voicemail on, turn on ARG3 in sp-sub-user, and add *97/*98 to
sp-featurecodes (which replaces the pending placeholder). Endpoints with a
mailbox get mwi_subscribe_replaces_unsolicited so phones that SUBSCRIBE for
MWI aren't refused with a 500. A new test:asterisk check leaves a message,
sees MWI on a baresip phone, plays it with *97 and sees MWI turn off.

Task: M4-T1

apps/api/test/asterisk/call-test.ts

@@ -48,6 +48,7 @@ function snapshot(): Snapshot {
     maxContacts: 5,
     ringSeconds: 25,
     voicemailEnabled: false,
+    voicemailPin: '4826',
     outboundClass: 'national' as const,
     outboundCallerId: null,
     emergencyCallerId: null,