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

configs/config.example.ini

@@ -28,6 +28,8 @@ path = data/baihu.db
 dsn = 
 # 表前缀
 table_prefix = baihu_
+# SSL 模式 (仅 mysql/postgres): postgres 支持 disable/require/verify-ca/verify-full; mysql 支持 true/skip-verify
+# ssl_mode = disable
 
 [security]
 # JWT 密钥。留空则在首次启动时自动生成并保存到数据库设置中。