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

package.json

@@ -15,7 +15,7 @@
     "lint": "eslint . && prettier --check .",
     "format": "prettier --write .",
     "typecheck": "pnpm -r typecheck",
-    "test:asterisk": "node apps/api/test/asterisk/load-golden.mjs && node --conditions=source apps/api/test/asterisk/call-test.ts && node --conditions=source apps/api/test/asterisk/trunk-test.ts"
+    "test:asterisk": "node apps/api/test/asterisk/load-golden.mjs && node --conditions=source apps/api/test/asterisk/call-test.ts && node --conditions=source apps/api/test/asterisk/trunk-test.ts && node --conditions=source apps/api/test/asterisk/voicemail-test.ts"
   },
   "devDependencies": {
     "@eslint/js": "^10.0.1",