announcements: play a sound, then continue

/api/announcements CRUD (sound must be a prompt, destination must exist
and not be itself, delete refused while referenced); the generator
renders sp-ann-{id} (Answer, Playback, destApp) and destApp sends
announcement destinations there. The ivr golden case gains "press 3:
hours announcement, then back to the menu". Announcements are managed in
a section of Sounds & music (05 updated); the DestinationPicker gains
"Announcement".

Task: M6-T5

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

@@ -104,6 +104,7 @@ function snapshot(): Snapshot {
     ivrs: [],
     timeConditions: [],
     holidays: [],
+    announcements: [],
     routes: [
       {
         id: 'aaaaaaaa-0000-4000-8000-000000000001',