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/notifications/repository.py

@@ -9,6 +9,7 @@ from sqlalchemy.ext.asyncio import AsyncSession
 
 from models.notification import Notification
 from models.user_notification import UserNotification
+from schemas.enums import NotificationTargetMode
 
 
 class NotificationRepository:
@@ -116,13 +117,24 @@ class NotificationRepository:
     async def commit(self) -> None:
         await self._session.commit()
 
-    async def link_published_notifications_to_user(self, *, user_id: UUID) -> int:
+    async def link_notifications_for_registered_user(
+        self, *, user_id: UUID, is_first_registration: bool
+    ) -> int:
+        target_modes: list[NotificationTargetMode]
+        if is_first_registration:
+            target_modes = [
+                NotificationTargetMode.NEW_USERS,
+                NotificationTargetMode.ALL_USERS,
+            ]
+        else:
+            target_modes = [NotificationTargetMode.ALL_USERS]
         notification_ids = list(
             (
                 await self._session.execute(
                     select(Notification.id).where(
                         Notification.status == "published",
                         Notification.deleted_at.is_(None),
+                        Notification.target_mode.in_(target_modes),
                     )
                 )
             )
@@ -136,8 +148,8 @@ class NotificationRepository:
             insert(UserNotification)
             .values(
                 [
-                    {"user_id": user_id, "notification_id": notification_id}
-                    for notification_id in notification_ids
+                    {"user_id": user_id, "notification_id": nid}
+                    for nid in notification_ids
                 ]
             )
             .on_conflict_do_nothing(index_elements=["user_id", "notification_id"])