- Add reminders_disabled boolean column to users (DB source of truth).
- Scheduler reads the DB column directly; Redis no longer used for
opt-out checks.
- Scheduler now deactivates reminders when a user has no completed
attempts left for a quiz (e.g., after deleting their attempts).
- Settings API: GET returns DB value for reminders_disabled; PUT
persists that key to DB and keeps the rest of the blob in Redis.
Rollback point for Alembic wiring.
b39e09f393