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/sns_share.py

@@ -0,0 +1,35 @@
+from sqlalchemy import Column, Integer, String, Float, DateTime, ForeignKey, Text
+from sqlalchemy.orm import relationship
+from sqlalchemy.sql import func
+from ..database import Base
+
+
+class SnsShareSubmission(Base):
+    """SNS 공유 제출 및 검증 모델"""
+    __tablename__ = "sns_share_submissions"
+
+    id = Column(Integer, primary_key=True, index=True)
+    user_id = Column(Integer, ForeignKey("users.id"), nullable=False)
+    car_id = Column(Integer, ForeignKey("cars.id"), nullable=False)
+
+    # SNS 정보
+    platform = Column(String(20), nullable=False)  # twitter, instagram, facebook
+    sns_url = Column(String(500), nullable=False)  # 공유 게시물 URL
+
+    # 상태
+    status = Column(String(20), default="pending")  # pending, approved, rejected
+    rejected_reason = Column(Text, nullable=True)  # 거부 사유
+
+    # CC 보상
+    reward_cc = Column(Float, default=3.0)  # 기본 3CC 보상
+    rewarded_at = Column(DateTime(timezone=True), nullable=True)  # 보상 지급 시각
+
+    # 타임스탬프
+    submitted_at = Column(DateTime(timezone=True), server_default=func.now())
+    verified_at = Column(DateTime(timezone=True), nullable=True)  # 검증 시각
+    verified_by = Column(Integer, ForeignKey("users.id"), nullable=True)  # 검증한 관리자
+
+    # Relationships
+    user = relationship("User", foreign_keys=[user_id], backref="sns_submissions")
+    car = relationship("Car", backref="sns_shares")
+    verifier = relationship("User", foreign_keys=[verified_by])