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/src/generator/load.ts

@@ -17,6 +17,7 @@ export type SnapExtension = {
   maxContacts: number;
   ringSeconds: number;
   voicemailEnabled: boolean;
+  voicemailPin: string; // decrypted
   outboundClass: 'internal' | 'local' | 'national' | 'international';
   outboundCallerId: string | null; // DID number
   emergencyCallerId: string | null; // DID number
@@ -147,6 +148,7 @@ export async function loadSnapshot(db: Db, now = new Date()): Promise<Snapshot>
       maxContacts: ext.maxContacts,
       ringSeconds: ext.ringSeconds,
       voicemailEnabled: ext.voicemailEnabled,
+      voicemailPin: decrypt(ext.voicemailPinEnc),
       outboundClass: ext.outboundClass,
       outboundCallerId: outCid,
       emergencyCallerId: e911Cid,