Step 3：Study / Site / StudyMember + 项目权限

backend/app/models/study_member.py

@@ -0,0 +1,20 @@
+import uuid
+from datetime import datetime
+
+from sqlalchemy import Boolean, DateTime, ForeignKey, String, UniqueConstraint, func
+from sqlalchemy.dialects.postgresql import UUID
+from sqlalchemy.orm import Mapped, mapped_column
+
+from app.db.base_class import Base
+
+
+class StudyMember(Base):
+    __tablename__ = "study_members"
+    __table_args__ = (UniqueConstraint("study_id", "user_id", name="uq_study_member"),)
+
+    id: Mapped[uuid.UUID] = mapped_column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
+    study_id: Mapped[uuid.UUID] = mapped_column(UUID(as_uuid=True), ForeignKey("studies.id"), nullable=False, index=True)
+    user_id: Mapped[uuid.UUID] = mapped_column(UUID(as_uuid=True), ForeignKey("users.id"), nullable=False)
+    role_in_study: Mapped[str] = mapped_column(String(20), nullable=False)
+    is_active: Mapped[bool] = mapped_column(Boolean, nullable=False, server_default="true")
+    added_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), nullable=False, server_default=func.now())