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/agent.py

@@ -16,11 +16,14 @@ class Agent(Base):
     name = Column(String(100), nullable=False, comment="智能体名称")
     description = Column(Text, comment="描述")
     workflow_config = Column(JSON, nullable=False, comment="工作流配置")
+    agent_type = Column(String(20), default="specialist", comment="Agent类型: main/specialist")
     budget_config = Column(
         JSON,
         nullable=True,
         comment="执行预算：max_steps/max_llm_invocations/max_tool_calls（可选，覆盖全局默认）",
     )
+    input_schema = Column(JSON, nullable=True, comment="输入数据类型约束（JSON Schema）")
+    output_schema = Column(JSON, nullable=True, comment="输出数据类型约束（JSON Schema）")
     version = Column(Integer, default=1, comment="版本号")
     status = Column(String(20), default="draft", comment="状态: draft/published/running/stopped")
     user_id = Column(CHAR(36), ForeignKey("users.id"), comment="创建者ID")