feat(database): support configurable SSL mode for PostgreSQL and MySQL

Add ssl_mode configuration option to allow users to customize SSL/TLS
policy for database connections instead of hardcoding sslmode=disable
for PostgreSQL and having no TLS support for MySQL.

Changes:
- database.Config, DatabaseConfig: add SSLMode field
- PostgreSQL DSN: replace hardcoded sslmode=disable with configurable
value
- MySQL DSN: append &tls=<SSLMode> parameter when SSLMode is set
- Full config pipeline: INI/env -> DatabaseConfig -> Config -> DSN ->
subprocess
- New env var BH_DB_SSL_MODE and ini key [database] ssl_mode
- Updated config example and documentation

docs/guide/configuration.md

@@ -22,6 +22,7 @@
 | `BH_DB_PATH` | database.path | SQLite 物理文件存储路径 | ./data/baihu.db |
 | `BH_DB_DSN` | database.dsn | 数据库 DSN (仅 mysql/postgres, 优先级高。**需对应设置 type**) | - |
 | `BH_DB_TABLE_PREFIX` | database.table_prefix | 数据库表前缀 | baihu_ |
+| `BH_DB_SSL_MODE` | database.ssl_mode | SSL 模式: postgres 支持 disable/require/verify-ca/verify-full; mysql 支持 true/skip-verify | - |
 | `BAIHU_SECRET_KEY` | - | 系统加密秘钥，用于机密变量功能（**注：仅支持环境变量设置，不支持配置文件**） | - |
 
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