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/points/service.py

@@ -61,6 +61,7 @@ class RegisterBonusResult:
     amount: int
     balance_after: int
     event_id: str
+    is_first_registration: bool = False
 
 
 @dataclass(frozen=True)
@@ -122,14 +123,17 @@ class PointsService:
         account = await self._repository.get_or_create_user_points_for_update(
             user_id=user_id
         )
-        if claim is not None and claim.balance_snapshot is not None:
-            account.balance = max(int(claim.balance_snapshot), 0)
-            account.version = int(account.version) + 1
+        if claim is not None:
+            is_first_registration = claim.first_user_id_snapshot is None
+            if claim.balance_snapshot is not None:
+                account.balance = max(int(claim.balance_snapshot), 0)
+                account.version = int(account.version) + 1
             return RegisterBonusResult(
                 granted=False,
                 amount=0,
                 balance_after=int(account.balance),
                 event_id=event_id,
+                is_first_registration=is_first_registration,
             )
 
         claimed = await self._repository.claim_register_bonus(
@@ -144,6 +148,7 @@ class PointsService:
                 amount=0,
                 balance_after=int(account.balance),
                 event_id=event_id,
+                is_first_registration=False,
             )
 
         balance = int(account.balance)
@@ -197,6 +202,7 @@ class PointsService:
             amount=bonus_points,
             balance_after=int(account.balance),
             event_id=event_id,
+            is_first_registration=True,
         )
 
     async def ensure_run_points_available(