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/referral.py

@@ -16,10 +16,13 @@ class ReferralReward(Base):
     # 피추천인 (추천받아 가입한 사람)
     referred_user_id = Column(Integer, ForeignKey("users.id"), nullable=False, index=True)
 
-    # 결제 금액 (피추천인이 충전한 금액 USD)
-    payment_amount = Column(Float, nullable=False)
+    # 보상 유형: "signup" (가입 시), "payment" (결제 시)
+    reward_type = Column(String(20), default="payment")
 
-    # 보상 금액 (결제 금액의 X%)
+    # 결제 금액 (피추천인이 충전한 금액 USD, signup인 경우 0)
+    payment_amount = Column(Float, default=0.0)
+
+    # 보상 금액 (결제 금액의 X% 또는 고정 CC)
     reward_amount = Column(Float, nullable=False)
 
     # 보상 상태: pending(대기), credited(적립), withdrawn(출금)