Add FastAPI backend with agent system

backend/app/models/task.py

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+from sqlalchemy import Column, String, Text, Integer, ForeignKey, DateTime, Enum
+from sqlalchemy.orm import relationship
+from datetime import datetime
+from enum import Enum as PyEnum
+from app.models.base import BaseModel
+
+
+class TaskStatus(str, PyEnum):
+    TODO = "todo"
+    IN_PROGRESS = "in_progress"
+    DONE = "done"
+    CANCELLED = "cancelled"
+
+
+class TaskPriority(str, PyEnum):
+    LOW = "low"
+    MEDIUM = "medium"
+    HIGH = "high"
+    URGENT = "urgent"
+
+
+class Task(BaseModel):
+    __tablename__ = "tasks"
+
+    user_id = Column(String(36), ForeignKey("users.id"), nullable=False, index=True)
+    title = Column(String(500), nullable=False)
+    description = Column(Text, nullable=True)
+    status = Column(Enum(TaskStatus), default=TaskStatus.TODO, nullable=False, index=True)
+    priority = Column(Enum(TaskPriority), default=TaskPriority.MEDIUM, nullable=False)
+    due_date = Column(DateTime, nullable=True)
+    completed_at = Column(DateTime, nullable=True)
+    tags = Column(String(1000), nullable=True)  # JSON 数组
+
+    history = relationship("TaskHistory", back_populates="task", cascade="all, delete-orphan")
+
+
+class TaskHistory(BaseModel):
+    __tablename__ = "task_histories"
+
+    task_id = Column(String(36), ForeignKey("tasks.id"), nullable=False, index=True)
+    action = Column(String(100), nullable=False)  # created, status_changed, updated, deleted
+    old_value = Column(Text, nullable=True)
+    new_value = Column(Text, nullable=True)
+
+    task = relationship("Task", back_populates="history")