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/src/generator/golden-case.ts

@@ -21,6 +21,7 @@ export function loadCase(name: string): Snapshot {
     ivrs?: Partial<Snapshot['ivrs'][number]>[];
     timeConditions?: Partial<Snapshot['timeConditions'][number]>[];
     holidays?: Snapshot['holidays'];
+    announcements?: Snapshot['announcements'];
   };
   const settings = Object.fromEntries(
     (Object.keys(SettingsSchemas) as SettingKey[])
@@ -52,6 +53,7 @@ export function loadCase(name: string): Snapshot {
       id: t.id!,
     })),
     holidays: raw.holidays ?? [],
+    announcements: raw.announcements ?? [],
     ivrs: (raw.ivrs ?? []).map((r, i) => ({
       name: `Menu ${i + 1}`,
       greeting: 'custom/greeting',