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/schemas/user.py

@@ -0,0 +1,26 @@
+from datetime import datetime
+from pydantic import BaseModel
+
+
+class UserCreate(BaseModel):
+    """创建用户"""
+    username: str
+    password: str
+    role: str = "submitter"  # admin=管理员, submitter=提交者
+
+
+class UserResponse(BaseModel):
+    """用户信息"""
+    id: int
+    username: str
+    role: str
+    created_at: datetime
+
+    class Config:
+        from_attributes = True
+
+
+class UserRegister(BaseModel):
+    """公开注册（仅限提交者）"""
+    username: str
+    password: str