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_register_bonus_first_registration.py

@@ -0,0 +1,125 @@
+from __future__ import annotations
+
+from uuid import uuid4
+
+import pytest
+
+from models.register_bonus_claims import RegisterBonusClaims
+from v1.points.service import PointsService
+
+
+class _FakeAccount:
+    balance: int = 100
+    frozen_balance: int = 0
+    lifetime_earned: int = 0
+    lifetime_spent: int = 0
+    version: int = 0
+
+
+class _FakePointsRepository:
+    def __init__(self, *, claim: RegisterBonusClaims | None = None) -> None:
+        self.account = _FakeAccount()
+        self.claim = claim
+        self.claimed = False
+        self.appended_ledger: list[object] = []
+        self.appended_audit: list[object] = []
+
+    async def get_or_create_user_points_for_update(
+        self, *, user_id: object
+    ) -> _FakeAccount:
+        return self.account
+
+    async def has_ledger_event(self, *, user_id: object, event_id: str) -> bool:
+        return False
+
+    async def append_ledger(self, *, command: object, balance_after: int) -> None:
+        self.appended_ledger.append(command)
+
+    async def append_audit_ledger(self, *, command: object) -> None:
+        self.appended_audit.append(command)
+
+    async def has_audit_event(self, *, event_id: str) -> bool:
+        return False
+
+    async def claim_register_bonus(
+        self,
+        *,
+        email_hash: str,
+        user_email_snapshot: str,
+        first_user_id_snapshot: object,
+        grant_event_id: str,
+    ) -> bool:
+        if self.claimed:
+            return False
+        self.claimed = True
+        return True
+
+    async def get_register_bonus_claim(
+        self, *, email_hash: str
+    ) -> RegisterBonusClaims | None:
+        return self.claim
+
+
+class TestRegisterBonusIsFirstRegistration:
+    @pytest.mark.asyncio
+    async def test_first_registration_sets_true(self) -> None:
+        repo = _FakePointsRepository(claim=None)
+        service = PointsService(repository=repo)  # type: ignore[arg-type]
+
+        result = await service.grant_register_bonus_if_eligible(
+            user_id=uuid4(), user_email="new@example.com"
+        )
+
+        assert result.granted is True
+        assert result.is_first_registration is True
+
+    @pytest.mark.asyncio
+    async def test_reregistered_with_existing_claim_sets_false(self) -> None:
+        existing_claim = RegisterBonusClaims(
+            email_hash="abc",
+            user_email_snapshot="old@example.com",
+            first_user_id_snapshot=uuid4(),
+            balance_snapshot=50,
+            grant_event_id="evt",
+        )
+        repo = _FakePointsRepository(claim=existing_claim)
+        service = PointsService(repository=repo)  # type: ignore[arg-type]
+
+        result = await service.grant_register_bonus_if_eligible(
+            user_id=uuid4(), user_email="old@example.com"
+        )
+
+        assert result.granted is False
+        assert result.is_first_registration is False
+
+    @pytest.mark.asyncio
+    async def test_reregistered_claim_without_first_user_id_sets_true(self) -> None:
+        claim_no_snapshot = RegisterBonusClaims(
+            email_hash="abc",
+            user_email_snapshot="edge@example.com",
+            first_user_id_snapshot=None,
+            balance_snapshot=50,
+            grant_event_id="evt",
+        )
+        repo = _FakePointsRepository(claim=claim_no_snapshot)
+        service = PointsService(repository=repo)  # type: ignore[arg-type]
+
+        result = await service.grant_register_bonus_if_eligible(
+            user_id=uuid4(), user_email="edge@example.com"
+        )
+
+        assert result.granted is False
+        assert result.is_first_registration is True
+
+    @pytest.mark.asyncio
+    async def test_claim_competition_failure_sets_false(self) -> None:
+        repo = _FakePointsRepository(claim=None)
+        repo.claimed = True
+        service = PointsService(repository=repo)  # type: ignore[arg-type]
+
+        result = await service.grant_register_bonus_if_eligible(
+            user_id=uuid4(), user_email="race@example.com"
+        )
+
+        assert result.granted is False
+        assert result.is_first_registration is False