feat: Cloud Functions — leaveCoupleCallable, onUserDelete cascade (batch v0.2.8)
- Add leaveCoupleCallable: HTTPS callable that atomically unlinks couple via Admin SDK (clears both user coupleIds, recursiveDelete couple doc)
- Add onUserDelete: Auth deletion trigger that cascades cleanup — unpairs partner, sends FCM notification, deletes Storage objects, recursiveDelete user doc
- Replace client-side batch leaveCouple with callable invocation (Firestore rules prevent cross-user writes)
- Remove CoupleRepository/UserRepository from DeleteAccountViewModel — cleanup now handled by onUserDelete trigger
- Wire new functions into index.ts exports
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