feat: add Goal/Task data models, service layer, and API routes (Phase 1)

Main Agent 数字员工工厂基础设施：
- 新增 Goal 和 Task SQLAlchemy 数据模型
- Agent 模型新增 agent_type / input_schema / output_schema
- Execution 模型新增 goal_id 关联
- 新增 Goal/Task CRUD 服务层（含依赖检查、任务树、进度计算）
- 新增 /api/v1/goals (9端点) + /api/v1/tasks (8端点)
- 数据库迁移 013_add_goals_tasks

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/models/task.py

@@ -0,0 +1,59 @@
+"""
+任务模型 — Goal 拆解的子任务
+"""
+from sqlalchemy import Column, String, Text, Integer, DateTime, JSON, Boolean, ForeignKey, func
+from sqlalchemy.dialects.mysql import CHAR
+from sqlalchemy.orm import relationship
+from app.core.database import Base
+import uuid
+
+
+class Task(Base):
+    """任务表 — Main Agent 将目标分解为可执行的子任务"""
+    __tablename__ = "tasks"
+
+    id = Column(CHAR(36), primary_key=True, default=lambda: str(uuid.uuid4()), comment="任务ID")
+    goal_id = Column(CHAR(36), ForeignKey("goals.id"), nullable=False, comment="所属目标ID")
+    title = Column(String(500), nullable=False, comment="任务标题")
+    description = Column(Text, comment="任务描述")
+    status = Column(
+        String(20), default="pending",
+        comment="状态: pending/in_progress/awaiting_approval/completed/failed/cancelled"
+    )
+    priority = Column(Integer, default=5, comment="优先级 1-10")
+
+    # 任务编排配置
+    task_config = Column(JSON, comment="编排配置: {orchestration_mode, agents:[], workflow_id, input_data}")
+
+    # 依赖关系
+    parent_task_id = Column(CHAR(36), ForeignKey("tasks.id"), nullable=True, comment="父任务ID")
+    depends_on = Column(JSON, default=list, comment="前置依赖任务ID列表")
+
+    # 执行结果
+    result = Column(JSON, comment="执行输出结果")
+    error_message = Column(Text, comment="错误信息")
+    execution_id = Column(CHAR(36), ForeignKey("executions.id"), nullable=True, comment="关联的执行记录ID")
+
+    # 分配
+    assigned_agent_id = Column(CHAR(36), ForeignKey("agents.id"), nullable=True, comment="分配的 Agent ID")
+    assigned_agent_name = Column(String(200), comment="分配的 Agent 名称（冗余便于展示）")
+
+    # 审批
+    requires_approval = Column(Boolean, default=False, comment="是否需要人工审批")
+    approver_id = Column(CHAR(36), ForeignKey("users.id"), nullable=True, comment="审批人ID")
+    approval_status = Column(String(20), comment="审批状态: pending/approved/rejected")
+
+    # 时间
+    started_at = Column(DateTime, comment="开始时间")
+    completed_at = Column(DateTime, comment="完成时间")
+    deadline = Column(DateTime, comment="截止时间")
+    created_at = Column(DateTime, default=func.now(), comment="创建时间")
+    updated_at = Column(DateTime, default=func.now(), onupdate=func.now(), comment="更新时间")
+
+    # 关系
+    goal = relationship("Goal", backref="tasks")
+    assigned_agent = relationship("Agent", backref="assigned_tasks")
+    execution = relationship("Execution", backref="task")
+
+    def __repr__(self):
+        return f"<Task(id={self.id}, title={self.title}, status={self.status})>"