fix(parser): add native Douyin detail fallback

[goxofy]

Summary

• Add a native fallback to Douyin's official Web detail endpoint when share-page HTML no longer embeds the work details.
• Require a deployment-owned PARSE_VIDEO_DOUYIN_COOKIE; no third-party parsing service or hard-coded credential is used.
• Add local signing, response validation, and actionable official status errors.
• Document the optional deployment configuration and credential-handling boundary.

Validation

• go test ./...
• go vet ./...
• go build ./...
• docker build -t parse-video:douyin-native-recovery-04f1708 .
• Verified parsing against an authorized, currently public Douyin share link.

Security

• No real cookie, session identifier, or authorization value is included in this branch.
• The configured cookie is sent only to the official www.douyin.com detail endpoint.

🤖 Generated with Claude Code

[wujunwei928]

已在独立 worktree 完成验证：

• go build / go vet / 新增 6 个单测全过（SM3 标准向量、a_bogus 确定性、query 顺序锁定、mock 请求构造、官方错误透传、无 cookie 拒绝请求）
• 原有抖音测试不回归；图集（isNote）路径不受影响
• 真实解析对比：空壳分享页场景下，main 报模糊错误 get video info fail: - ，本 PR 无 cookie 时返回明确的配置错误，带 cookie 时正确构造含 a_bogus 的签名请求
• 主路径（分享页含详情）不受影响，后备仅在数据缺失时触发

备注：本机网络无法访问 www.douyin.com，带真实 Cookie 的端到端签名验证依赖作者声明，建议合并后在有 Cookie 的部署环境再实测一次。两个不阻塞的小瑕疵：douyinABogusUserAgentCode 32 字节表仅用 2 个元素；UA 写 Chrome/90 但 query 里 browser_version=130，指纹不一致或增加风控风险。