[user-service] migrate_firebase_users task in tasks_executor

[davidgamez]

Describe the problem

This issue adds a task to import Firebase users into the DB schema.

Proposed solution

The created task should be idempotent. It reads all Firestore user records via the Firebase Admin SDK and upserts users.app_user (skip existing by firebase_uid), stores raw org string in legacy_org_name. Supports dry_run. Safe to run repeatedly during the deployment window.
Field mapping:

DB field	Resolve
users.id	Datastore: uid
users.full_name	Datastore: fullName
users.legacy_org_name	Datastore: organization
notifications => users.notification_subscription linked to api. announcements	Datastore: isRegisteredToReceiveAPIAnnouncements
users.registration_completed_at	Datastore: registrationCompletionTime
user.email_verified	Firestore DSK email_verified
users.created_at	Firebase SDK createdAt
users.updated_at	now

Alternatives you've considered

No response

Additional context

No response