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.

docs/protocols/common/user-points-chat-data-protocol.md

@@ -4,7 +4,7 @@ This protocol defines the canonical data contract for user profile, points accou
 
 Protocol verification status:
 
-- Last audited migration: `backend/alembic/versions/20260411_0003_points_audit_and_register_bonus_claims.py`
+- Last audited migration: `backend/alembic/versions/20260413_0004_register_bonus_claims_snapshot.py`
 - Last audited models: `backend/src/models/profile.py`, `backend/src/models/user_points.py`, `backend/src/models/points_ledger.py`, `backend/src/models/points_audit_ledger.py`, `backend/src/models/register_bonus_claims.py`, `backend/src/models/agent_chat_session.py`, `backend/src/models/agent_chat_message.py`
 - Current status: aligned with register bonus moved to application service
 
@@ -95,13 +95,14 @@ Protocol verification status:
 ### register_bonus_claims
 
 - PK: `id`
-- Core fields: `email_hash`, `user_email_snapshot`, `first_user_id`, `grant_event_id`, `created_at`, `updated_at`
+- Core fields: `email_hash`, `user_email_snapshot`, `first_user_id_snapshot`, `balance_snapshot`, `grant_event_id`, `created_at`, `updated_at`
 - Constraints:
   - `email_hash` unique
   - `grant_event_id` unique
 - Notes:
   - `email_hash` must be HMAC-SHA256 over normalized email (`trim + lower`)
   - key source: backend config `points_policy.register_bonus_hmac_key`
+  - `balance_snapshot` stores the latest pre-delete account balance for same-email re-registration recovery
 
 #### points_ledger.metadata (schema_version=1)
 
@@ -145,7 +146,7 @@ JSON constraints:
   - Function: `public.initialize_profile_and_invite_code_on_signup()`
   - Side effects: profile init + invite code init
 - Application service (in `POST /auth/email-session`):
-  - `grant_register_bonus_if_eligible()` grants register bonus via `register_bonus_claims` ledger
+  - `grant_register_bonus_if_eligible()` restores `balance_snapshot` first when present; otherwise grants register bonus via `register_bonus_claims`
   - Bonus amount from `config.points_policy.register_bonus_points`
 
 ### sessions