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/dialplan/static.ts

@@ -88,6 +88,15 @@ exten => _X!,1,Answer()                ; X! also matches "0" (caller unknown)
  same => n,SayDigits(\${EXTEN})
  same => n,Hangup()
 
+; ─── Feature codes (03 §6.13). M4: voicemail only; M6-T7 adds the rest.
+[sp-featurecodes]
+exten => *97,1,Answer()                                     ; my voicemail
+ same => n,VoiceMailMain(\${SP_EXT}@default)
+ same => n,Hangup()
+exten => *98,1,Answer()                                     ; any voicemail (asks mailbox)
+ same => n,VoiceMailMain(@default)
+ same => n,Hangup()
+
 [sp-vm-direct]
 exten => _X.,1,Answer()
  same => n,VoiceMail(\${EXTEN}@default,u)