Add default sound to iOS push payload

[requilence]

Summary

• iOS push notifications arrive silently. The APNs payload built by buildFcmIosMessage sets only mutable-content: 1, and the Firebase Admin SDK omits the sound key entirely when Aps.Sound is empty — so the delivered notification has content.sound == nil and iOS plays nothing.
• Nothing sets it on the client either: the iOS notification service extension mutates body, userInfo and badge but never sound, and iOS has no per-app sound default to fall back on. Android is unaffected because it receives a data-only message and builds the notification itself, where the NotificationChannel supplies the sound.
• Setting it server-side rather than in the extension is deliberate: the extension is best-effort. On decryption failure, a missing encryption key, timeout or crash, the system falls back to the original payload — so the payload has to carry the sound for those paths to make any noise.

Payload before/after, verified against the Firebase SDK marshaller:

before: {"mutable-content":1}
after:  {"mutable-content":1,"sound":"default"}

buildFcmIosSilentMessage is intentionally untouched — that is the NotifyRead background push used to dismiss delivered notifications on other devices, and it must stay content-available only.

[github-actions]

New Coverage	34.2% of statements
Patch Coverage	0.0% of changed statements (0/1)

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