feat: searxng.toml 支持 auth_basic/auth_bearer 认证配置

- common.py: resolve_auth_basic/bearer 新增 config_value 参数，优先级 CLI > file > config > env
- search.py: main() 从 load_config() 读取 auth_basic/auth_bearer；修复 --config 指定文件中 instance 字段不被解析的问题
- LICENSE: 补齐 MIT 协议文件
- tests: +21 测试覆盖配置文件认证优先级链与 main() 集成（309→330）
- docs: SKILL.md/README.md 同步更新认证配置说明与安全提醒

LICENSE

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+MIT License
+
+Copyright (c) 2026 Metona Team
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+
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