refactor: 法治日报改为法治网 + 删除无用conversations/messages表

- 删除 conversations 和 messages 表（未被代码使用）
- 迁移文件：000017_drop_conversations_messages.up/down.sql
- 种子文件改名：seed_fazhiribao_* → seed_fazhiwang_*
- 组织数据：法治日报 → 法治网（slug: fazhiwang）
- 用户数据：法治日报管理员等 → 法治网管理员等
- 演示数据重构：使用 app_usage_logs 而非 conversations/messages
- 本地数据已更新：1个组织、15个应用、3个用户、5条示例对话

server/migrations/000017_drop_conversations_messages.down.sql

@@ -0,0 +1,24 @@
+-- 回滚：重新创建 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),
+    created_at TIMESTAMP WITH TIME ZONE DEFAULT CURRENT_TIMESTAMP,
+    updated_at TIMESTAMP WITH TIME ZONE DEFAULT CURRENT_TIMESTAMP
+);
+
+-- 回滚：重新创建 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(20) NOT NULL CHECK (role IN ('user', 'assistant', 'system')),
+    content TEXT NOT NULL,
+    created_at TIMESTAMP WITH TIME ZONE DEFAULT CURRENT_TIMESTAMP
+);
+
+-- 回滚：创建索引
+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_messages_conversation_id ON messages(conversation_id);
+CREATE INDEX IF NOT EXISTS idx_messages_created_at ON messages(created_at);