移除用户系统级角色，权限完全迁移至项目级管理

- 用户注册/创建不再需要选择角色，账号管理只管资料和状态
- 移除前后端所有系统级 role 字段的暴露，改用 is_admin 标识管理员
- PM（项目负责人）角色自动拥有全部项目权限且不可修改
- 系统管理员在项目中的成员身份不可被删除
- 管理界面全面美化

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/app/schemas/user.py

@@ -6,7 +6,6 @@ from typing import Literal, Optional
 from pydantic import BaseModel, ConfigDict, EmailStr, Field, field_validator
 
 UserRole = Literal["PM", "CRA", "PV", "MEDICAL_REVIEW", "IMP", "QA", "ADMIN"]
-RegisterRole = Literal["PM", "CRA", "PV", "MEDICAL_REVIEW", "IMP"]
 UserStatus = Literal["PENDING", "ACTIVE", "REJECTED", "DISABLED"]
 
 PASSWORD_REGEX = re.compile(r"^(?=.*[A-Za-z])(?=.*\d).{8,}$")
@@ -16,7 +15,6 @@ class UserDisplay(BaseModel):
     id: uuid.UUID
     email: EmailStr
     full_name: str
-    role: UserRole
     avatar_url: Optional[str] = None
 
     model_config = ConfigDict(from_attributes=True)
@@ -39,7 +37,6 @@ class UserRegisterRequest(_PasswordValidator):
     password: str = Field(min_length=8, max_length=72)
     email: EmailStr
     full_name: str = Field(min_length=1)
-    role: RegisterRole
     clinical_department: str = Field(min_length=1)
 
 
@@ -47,7 +44,6 @@ class UserCreate(_PasswordValidator):
     password: str = Field(min_length=8, max_length=72)
     email: EmailStr
     full_name: str = Field(min_length=1)
-    role: UserRole
     clinical_department: str = Field(min_length=1)
     status: UserStatus = "ACTIVE"
 
@@ -57,10 +53,10 @@ class UserRead(BaseModel):
     email: EmailStr
     username: str
     full_name: str
-    role: UserRole
     clinical_department: str
     status: UserStatus
     is_active: bool
+    is_admin: bool = False
     created_at: datetime
     approved_at: Optional[datetime] = None
     approved_by: Optional[uuid.UUID] = None
@@ -72,7 +68,6 @@ class UserRead(BaseModel):
 class UserUpdate(_PasswordValidator):
     email: Optional[EmailStr] = None
     full_name: Optional[str] = None
-    role: Optional[UserRole] = None
     clinical_department: Optional[str] = None
     status: Optional[UserStatus] = None
     password: Optional[str] = None