feat: Add banner toggle and soldout tracking to Cars page

- Add is_banner, soldout fields to Car model
- Add banner toggle API (POST /hero-banners/admin/toggle/{car_id})
- Add soldout APIs (POST/DELETE /cars/{car_id}/soldout)
- Add nightly soldout checker in agent (runs at 3:00 AM)
- Update Local Cars UI with banner checkbox and status column
- Remove hero-banners admin page (functionality moved to Cars page)
- Banner cars sorted to top with purple background
- Soldout cars displayed with gray overlay

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

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

backend/app/models/car.py

@@ -50,6 +50,8 @@ class Car(Base):
     margin_mn = Column(BigInteger, default=0)   # Mongolian margin amount in KRW
     price_usd = Column(DECIMAL(12, 2))
     is_displayed = Column(Boolean, default=False, index=True)  # Show to users
+    is_banner = Column(Boolean, default=False, index=True)  # Registered as hero banner
+    soldout = Column(Boolean, default=False, index=True)  # Sold to another buyer (checked nightly vs Carmodoo)
 
     fuel = Column(String(20))
     transmission = Column(String(20))