fix: preserve points balance across account re-registration

Persist a per-email balance snapshot before account deletion and restore it on same-email re-registration, preventing both unintended balance reset and repeated signup bonus grants.

backend/src/models/register_bonus_claims.py

@@ -2,7 +2,7 @@ from __future__ import annotations
 
 import uuid
 
-from sqlalchemy import ForeignKey, String, Text, UniqueConstraint
+from sqlalchemy import BigInteger, String, Text, UniqueConstraint
 from sqlalchemy.dialects.postgresql import UUID
 from sqlalchemy.orm import Mapped, mapped_column
 
@@ -25,9 +25,8 @@ class RegisterBonusClaims(TimestampMixin, Base):
     )
     email_hash: Mapped[str] = mapped_column(String(64), nullable=False)
     user_email_snapshot: Mapped[str] = mapped_column(Text, nullable=False)
-    first_user_id: Mapped[uuid.UUID | None] = mapped_column(
-        UUID(as_uuid=True),
-        ForeignKey("auth.users.id", ondelete="SET NULL"),
-        nullable=True,
+    first_user_id_snapshot: Mapped[uuid.UUID | None] = mapped_column(
+        UUID(as_uuid=True), nullable=True
     )
+    balance_snapshot: Mapped[int | None] = mapped_column(BigInteger, nullable=True)
     grant_event_id: Mapped[str] = mapped_column(String(64), nullable=False)