feat: add user role system and public joke submission API

- Add AdminUser role field (admin/submitter)
- User management in admin panel
- Public joke submission API (no auth required)
- Document submission methods for external workers

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

api/app/models/user.py

@@ -5,10 +5,11 @@ from app.database import Base
 
 
 class AdminUser(Base):
-    """管理员用户表模型"""
+    """用户表 - 支持角色：admin(管理员), submitter(提交者)"""
     __tablename__ = "admin_users"
 
     id = Column(Integer, primary_key=True, autoincrement=True)
     username = Column(String(50), unique=True, nullable=False)
     password_hash = Column(String(255), nullable=False)
+    role = Column(String(20), default="submitter")          # admin=管理员, submitter=提交者
     created_at = Column(DateTime, default=func.now())