fix: prevent invite code enumeration via Cloud Function (batch v0.2.18)
- Remove client-side read access to invites (only inviter can read own invite)
- Deny direct client update to invites (server-side only via Admin SDK)
- Add acceptInviteCallable Cloud Function: validates code, creates couple,
updates user docs, marks invite accepted, returns wrapped key for local decryption
- Update Android client: FirestoreInviteDataSource calls callable function,
InviteConfirmViewModel uses acceptInvite + unwrapAndStore flow
- Deprecate CoupleRepositoryImpl.createCouple (client-side path removed)
- Update Firestore rules tests: unpaired read now denied, direct update now denied
- 118/118 tests passing
39255c8733