Step 5：里程碑与任务（伦理/启动会进度）

backend/app/models/task.py

@@ -0,0 +1,28 @@
+import uuid
+from datetime import datetime, date
+
+from sqlalchemy import Date, DateTime, ForeignKey, String, Text, func
+from sqlalchemy.dialects.postgresql import UUID
+from sqlalchemy.orm import Mapped, mapped_column
+
+from app.db.base_class import Base
+
+
+class Task(Base):
+    __tablename__ = "tasks"
+
+    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"), index=True, nullable=False)
+    milestone_id: Mapped[uuid.UUID | None] = mapped_column(UUID(as_uuid=True), ForeignKey("milestones.id"), nullable=True)
+    title: Mapped[str] = mapped_column(String(200), nullable=False)
+    description: Mapped[str | None] = mapped_column(Text, nullable=True)
+    assignee_id: Mapped[uuid.UUID | None] = mapped_column(UUID(as_uuid=True), ForeignKey("users.id"), nullable=True)
+    priority: Mapped[str] = mapped_column(String(20), nullable=False, default="MEDIUM")
+    due_date: Mapped[date | None] = mapped_column(Date, nullable=True)
+    status: Mapped[str] = mapped_column(String(20), nullable=False, default="TODO")
+    completed_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True)
+    created_by: Mapped[uuid.UUID] = mapped_column(UUID(as_uuid=True), ForeignKey("users.id"), nullable=False)
+    created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), nullable=False, server_default=func.now())
+    updated_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), nullable=False, server_default=func.now(), onupdate=func.now()
+    )