Enhance database configuration to support multiple data sources

- Updated .env.example to include optional configuration for additional database instances.
- Refactored DatabaseConfig in config.go to accommodate a default database and additional instances.
- Implemented loadAdditionalDBConfigs function to dynamically load configurations for extra databases.
- Modified InitDB function in database.go to establish connections for both default and additional databases.
- Updated README.md to document the new configuration options for connecting to multiple databases.

docs/README.md

@@ -27,6 +27,7 @@
 2. 按实际环境填写以下变量：
    - `SERVER_PORT`：HTTP 服务端口，例如 `8080`。
    - `DB_HOST/DB_PORT/DB_USER/DB_PASSWORD/DB_NAME`：MySQL 连接信息。
+   - 若需连接额外数据库，可设置 `DB_INSTANCES=lawyer,reporting` 并依次提供 `DB_<NAME>_HOST/PORT/USER/PASSWORD/NAME`；未指定的字段会回落到默认数据库的同名配置。
 3. 如果需要，替换 `GIN_MODE`、`JWT_SECRET` 等其他变量。
 4. 通过 `docs/sql/users.sql` 初始化 `mini_programs` 与 `users` 表，并插入每个小程序的 `name/app_id/app_secret`。