Extend the notification system with a third channel (telegram) alongside existing in_app and push channels. Add `telegram_enabled` column to `notification_preferences` table, update `NOTIFICATION_CHANNELS` and `_NOTIFICATION_CHANNEL_COLUMNS` mappings, and set telegram default to off (`_NOTIFICATION_CHANNEL_DEFAULTS`). Create `telegram_outbox` table with columns for uid, user_uid, chat_id, text, status, attempts, created_at, and sent_at, plus an index on status/id for efficient polling. Register `TelegramOutboxService` in the service manager lifecycle. Update API documentation strings to describe the new channel and its pairing requirement. Extend admin notification defaults view to include telegram column. Pass `notif_telegram_paired` flag to profile template based on `telegram_store.is_paired()` check. Update `NotificationPrefForm` and `NotificationDefaultForm` model literals to accept "telegram" as a valid channel value.
Implement configurable notification preferences across in-app and push channels, including a new `notification_preferences` table with soft-delete support, per-user toggle endpoints, admin defaults management, and canonical type definitions. The change introduces `NOTIFICATION_TYPES` and `NOTIFICATION_CHANNELS` constants, `NotificationPrefForm`/`NotificationDefaultForm` models, `notification_enabled` resolution logic, and UI integration via the profile notifications tab and admin panel.