feat(notification): add target_mode enum constraint and merge register-notifications script

- Add NotificationTargetMode enum (new_users/exist_users/all_users/user_ids)
- Create Alembic migrations: drop duplicate indexes, add target_mode column
- Merge register-notifications.sh into dev-migrate.sh sync-notifications subcommand
- Shorten notification config path: static/notification/notifications -> static/notifications
- Update registration flow to dispatch notifications by target_mode
- Add is_first_registration to RegisterBonusResult for first-time user detection
- Remove dead code: link_published_notifications_to_user
- Update welcome_points.yaml to target new_users only
- Add 44 unit tests + 1 integration test, all passing

backend/src/v1/auth/router.py

@@ -73,13 +73,14 @@ async def create_email_session(
     )
     result = await service.create_email_session(payload)
     points_service = PointsService(repository=PointsRepository(session))
-    await points_service.grant_register_bonus_if_eligible(
+    bonus_result = await points_service.grant_register_bonus_if_eligible(
         user_id=UUID(result.user.id),
         user_email=result.user.email,
     )
     notification_service = NotificationService(NotificationRepository(session))
-    linked_count = await notification_service.link_published_notifications_to_user(
-        user_id=UUID(result.user.id)
+    linked_count = await notification_service.link_notifications_for_registered_user(
+        user_id=UUID(result.user.id),
+        is_first_registration=bonus_result.is_first_registration,
     )
     await session.commit()
     logger.info(