feat:1.数据库存储功能添加（初版）2.后端REST API版本代码清理

backend/db/__init__.py

@@ -0,0 +1,25 @@
+"""
+AgentCoord 数据库模块
+提供 PostgreSQL 数据库连接、模型和 CRUD 操作
+基于 DATABASE_DESIGN.md 设计
+"""
+
+from .database import get_db, get_db_context, test_connection, engine, text
+from .models import MultiAgentTask, UserAgent, TaskStatus
+from .crud import MultiAgentTaskCRUD, UserAgentCRUD
+
+__all__ = [
+    # 连接管理
+    "get_db",
+    "get_db_context",
+    "test_connection",
+    "engine",
+    "text",
+    # 模型
+    "MultiAgentTask",
+    "UserAgent",
+    "TaskStatus",
+    # CRUD
+    "MultiAgentTaskCRUD",
+    "UserAgentCRUD",
+]