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/models/notification.py

@@ -9,6 +9,7 @@ from sqlalchemy.orm import Mapped, mapped_column
 
 from core.db.base import Base, SoftDeleteMixin, TimestampMixin
 from core.db.types import json_jsonb
+from schemas.enums import NotificationTargetMode
 
 
 class Notification(TimestampMixin, SoftDeleteMixin, Base):
@@ -18,6 +19,10 @@ class Notification(TimestampMixin, SoftDeleteMixin, Base):
             "status IN ('draft', 'published', 'revoked')",
             name="ck_notifications_status",
         ),
+        CheckConstraint(
+            "target_mode IN ('new_users', 'exist_users', 'all_users', 'user_ids')",
+            name="ck_notifications_target_mode",
+        ),
         CheckConstraint(
             "jsonb_typeof(payload) = 'object'",
             name="ck_notifications_payload_object",
@@ -63,6 +68,9 @@ class Notification(TimestampMixin, SoftDeleteMixin, Base):
     status: Mapped[str] = mapped_column(
         String(16), nullable=False, server_default=text("'published'")
     )
+    target_mode: Mapped[NotificationTargetMode] = mapped_column(
+        String(32), nullable=False, server_default=text("'all_users'")
+    )
     published_at: Mapped[datetime | None] = mapped_column(
         DateTime(timezone=True), nullable=True
     )