Migrate from Firebase Auth

Migrating from Firebase Authentication to AuthSetu can be completed without disrupting active users or requiring forced password resets.

Migration Strategies

1. **Scrypt Hash Import**: Export users using the Firebase CLI (`firebase auth:export`) and upload them via `/v1/users/import`. AuthSetu supports verifying legacy Firebase modified scrypt hashes.

2. **Lazy Migration / Dual-Check**: When a user signs in, if they do not yet exist in AuthSetu, authenticate them against Firebase, create their account in AuthSetu, and upgrade their password hash to Argon2id.

3. **Session Interceptor**: Redirect active Firebase tokens to AuthSetu's token exchange endpoint for zero-friction cutover.