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/service.py

@@ -123,9 +123,11 @@ class NotificationService:
             await self._repository.commit()
         return updated_count
 
-    async def link_published_notifications_to_user(self, *, user_id: UUID) -> int:
-        return await self._repository.link_published_notifications_to_user(
-            user_id=user_id
+    async def link_notifications_for_registered_user(
+        self, *, user_id: UUID, is_first_registration: bool
+    ) -> int:
+        return await self._repository.link_notifications_for_registered_user(
+            user_id=user_id, is_first_registration=is_first_registration
         )