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/tests/unit/test_static_notification_sync.py

@@ -10,6 +10,7 @@ from core.config.notification.static_sync import (
     build_static_notification_content_hash,
     load_static_notification_documents,
 )
+from schemas.enums import NotificationTargetMode
 
 
 def _write_yaml(path: Path, content: str) -> None:
@@ -43,10 +44,86 @@ def test_load_static_notification_file_parses_valid_yaml(tmp_path: Path) -> None
 
     assert loaded.notification.source_key == "welcome_bonus"
     assert loaded.notification.payload.action == "open_route"
-    assert loaded.targets.mode == "user_ids"
+    assert loaded.targets.mode == NotificationTargetMode.USER_IDS
     assert len(loaded.targets.user_ids or []) == 1
 
 
+def test_load_static_notification_file_parses_new_users(tmp_path: Path) -> None:
+    file_path = tmp_path / "welcome_points.yaml"
+    _write_yaml(
+        file_path,
+        """
+        notification:
+          source_key: welcome_points
+          version: 1
+          type: system
+          status: published
+          title: Welcome
+          body: You got points.
+          payload:
+            action: open_route
+            route: /points
+            tab: balance
+        targets:
+          mode: new_users
+        """,
+    )
+
+    loaded = load_static_notification_file(file_path)
+
+    assert loaded.targets.mode == NotificationTargetMode.NEW_USERS
+    assert loaded.targets.user_ids is None
+
+
+def test_load_static_notification_file_parses_exist_users(tmp_path: Path) -> None:
+    file_path = tmp_path / "promo.yaml"
+    _write_yaml(
+        file_path,
+        """
+        notification:
+          source_key: promo_return
+          version: 1
+          type: system
+          status: published
+          title: Come back
+          body: We miss you.
+          payload:
+            action: none
+        targets:
+          mode: exist_users
+        """,
+    )
+
+    loaded = load_static_notification_file(file_path)
+
+    assert loaded.targets.mode == NotificationTargetMode.EXIST_USERS
+    assert loaded.targets.user_ids is None
+
+
+def test_load_static_notification_file_parses_all_users(tmp_path: Path) -> None:
+    file_path = tmp_path / "announce.yaml"
+    _write_yaml(
+        file_path,
+        """
+        notification:
+          source_key: system_announce
+          version: 1
+          type: system
+          status: published
+          title: Announcement
+          body: Maintenance at midnight.
+          payload:
+            action: none
+        targets:
+          mode: all_users
+        """,
+    )
+
+    loaded = load_static_notification_file(file_path)
+
+    assert loaded.targets.mode == NotificationTargetMode.ALL_USERS
+
+
 def test_load_static_notification_file_rejects_invalid_targets(tmp_path: Path) -> None:
     file_path = tmp_path / "invalid.yaml"
     _write_yaml(
@@ -72,6 +149,81 @@ def test_load_static_notification_file_rejects_invalid_targets(tmp_path: Path) -
         load_static_notification_file(file_path)
 
 
+def test_load_static_notification_file_rejects_unknown_mode(tmp_path: Path) -> None:
+    file_path = tmp_path / "bad_mode.yaml"
+    _write_yaml(
+        file_path,
+        """
+        notification:
+          source_key: bad_mode
+          version: 1
+          type: system
+          status: published
+          title: Bad
+          body: Bad mode.
+          payload:
+            action: none
+        targets:
+          mode: non_existent
+        """,
+    )
+
+    with pytest.raises(ValueError, match="Invalid static notification data"):
+        load_static_notification_file(file_path)
+
+
+def test_load_static_notification_file_rejects_new_users_with_user_ids(
+    tmp_path: Path,
+) -> None:
+    file_path = tmp_path / "bad_new_users.yaml"
+    _write_yaml(
+        file_path,
+        """
+        notification:
+          source_key: bad_new
+          version: 1
+          type: system
+          status: published
+          title: Bad
+          body: Bad.
+          payload:
+            action: none
+        targets:
+          mode: new_users
+          user_ids:
+            - 11111111-1111-1111-1111-111111111111
+        """,
+    )
+
+    with pytest.raises(ValueError, match="Invalid static notification data"):
+        load_static_notification_file(file_path)
+
+
+def test_load_static_notification_file_rejects_user_ids_without_list(
+    tmp_path: Path,
+) -> None:
+    file_path = tmp_path / "bad_user_ids.yaml"
+    _write_yaml(
+        file_path,
+        """
+        notification:
+          source_key: bad_uids
+          version: 1
+          type: system
+          status: published
+          title: Bad
+          body: Bad.
+          payload:
+            action: none
+        targets:
+          mode: user_ids
+        """,
+    )
+
+    with pytest.raises(ValueError, match="Invalid static notification data"):
+        load_static_notification_file(file_path)
+
+
 def test_load_static_notification_documents_rejects_duplicate_source_key(
     tmp_path: Path,
 ) -> None: