feat: 添加 conversations 和 messages 表用于存储应用示例对话数据

- 创建 conversations 表（对话记录）
- 创建 messages 表（对话消息）
- 添加必要的索引和外键约束
- 为 seed_fazhiribao_demo_data.sql 准备表结构

server/migrations/000016_conversations_and_messages.down.sql

@@ -0,0 +1,5 @@
+-- 回滚：删除 messages 表
+DROP TABLE IF EXISTS messages;
+
+-- 回滚：删除 conversations 表
+DROP TABLE IF EXISTS conversations;

server/migrations/000016_conversations_and_messages.up.sql

@@ -0,0 +1,35 @@
+-- 创建对话（conversations）表
+CREATE TABLE IF NOT EXISTS conversations (
+    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    app_id UUID NOT NULL REFERENCES applications(id) ON DELETE CASCADE,
+    user_id UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    title VARCHAR(500) NOT NULL,
+    created_at TIMESTAMP NOT NULL DEFAULT NOW(),
+    updated_at TIMESTAMP NOT NULL DEFAULT NOW()
+);
+
+-- 创建消息（messages）表
+CREATE TABLE IF NOT EXISTS messages (
+    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    conversation_id UUID NOT NULL REFERENCES conversations(id) ON DELETE CASCADE,
+    role VARCHAR(50) NOT NULL CHECK (role IN ('user', 'assistant', 'system')),
+    content TEXT NOT NULL,
+    created_at TIMESTAMP NOT NULL DEFAULT NOW()
+);
+
+-- 创建索引以提升查询性能
+CREATE INDEX IF NOT EXISTS idx_conversations_app_id ON conversations(app_id);
+CREATE INDEX IF NOT EXISTS idx_conversations_user_id ON conversations(user_id);
+CREATE INDEX IF NOT EXISTS idx_conversations_created_at ON conversations(created_at DESC);
+CREATE INDEX IF NOT EXISTS idx_messages_conversation_id ON messages(conversation_id);
+CREATE INDEX IF NOT EXISTS idx_messages_created_at ON messages(created_at);
+
+-- 添加注释
+COMMENT ON TABLE conversations IS '应用对话记录表';
+COMMENT ON TABLE messages IS '对话消息表';
+COMMENT ON COLUMN conversations.app_id IS '关联的应用ID';
+COMMENT ON COLUMN conversations.user_id IS '发起对话的用户ID';
+COMMENT ON COLUMN conversations.title IS '对话标题';
+COMMENT ON COLUMN messages.conversation_id IS '所属对话ID';
+COMMENT ON COLUMN messages.role IS '消息角色：user/assistant/system';
+COMMENT ON COLUMN messages.content IS '消息内容';