MySQL init SQL fails with “No database selected” (missing USE)

## 描述
啟動 MySQL 容器時，初始化 SQL 執行失敗，錯誤為：
`ERROR 1046 (3D000) at line 4: No database selected`

初始化 SQL 內有 `CREATE DATABASE`，但沒有 `USE ` 或全限定表名，導致後續 `CREATE TABLE` 報錯。

## 影響檔案
`docker/volumes/mysql/schema.sql`

## 重現步驟
1. 使用乾淨資料目錄啟動 MySQL 容器（刪除現有 volume/資料夾）
2. 讓容器執行 `docker/volumes/mysql/schema.sql`
3. 觀察日誌

## 實際結果
初始化失敗，出現：
`ERROR 1046 (3D000) at line 4: No database selected`

## 預期結果
應該成功建立 schema 與所有表

## 建議修正
在 SQL 檔中加上：
```sql
CREATE DATABASE IF NOT EXISTS opencoze COLLATE utf8mb4_unicode_ci;
USE opencoze;

Open docker/volumes/mysql/schema.sql and inspect the initialization statements around the reported failure at line 4. Start by running the SQL against a clean MySQL data directory, then verify that initialization completes and the schema and all tables are created successfully.

Written by the indexing model from the issue text.