Add SNS Marketing Campaign feature

- Add cash_cc_balance to User model (withdrawable CC)
- Create SnsShareSubmission model for SNS share verification
- Add marketing campaign settings to SystemSettings
- Add reward_type to ReferralReward model
- Create /api/sns-share endpoints for submission and verification
- Add referral signup reward logic (10CC on signup)
- Create /sns-share user page for SNS sharing
- Create /admin/sns-shares management page
- Add marketing settings UI to admin settings page
- Add SNS Shares menu to admin sidebar

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/models/__init__.py

@@ -11,6 +11,7 @@ from .vehicle_share import VehicleShare, ShareReward
 from .withdrawal import WithdrawalRequest
 from .referral import ReferralReward
 from .notification import Notification
+from .sns_share import SnsShareSubmission
 from .push_subscription import PushSubscription, UserNotificationPreference
 from .performance_check import CarPerformanceCheck
 from .car_specification import CarSpecification
@@ -52,6 +53,7 @@ __all__ = [
     "WithdrawalRequest",
     "ReferralReward",
     "Notification",
+    "SnsShareSubmission",
     "PushSubscription",
     "UserNotificationPreference",
     "ExchangeRate",