fix(parser): restore cookie-free Douyin parsing with anonymous ttwid

[saberc8]

Summary

• Register an anonymous ttwid through ByteDance's official union endpoint when the share page no longer embeds videoInfoRes.
• Use that anonymous cookie for the existing signed official Douyin detail request, while preserving PARSE_VIDEO_DOUYIN_COOKIE as an optional deployment-owned override.
• Cover anonymous registration, configured-cookie behavior, and registration failures with tests, and update the related documentation.

Root cause

Douyin share pages still expose window._ROUTER_DATA, but no longer include videoInfoRes. The native detail fallback added in #100 reaches the correct official endpoint, but requires every deployment to provide an account Cookie, so a default installation still cannot parse current Douyin links.

Validation

• go test ./...
• go vet ./...
• go build ./...
• git diff --check
• Parsed https://v.douyin.com/SXBboRbpXC4/ with PARSE_VIDEO_DOUYIN_COOKIE unset; verified the expected author/title/cover and a non-empty video URL.
• Verified the returned media URL with a browser User-Agent: HTTP 206, video/mp4.

Security

• Uses only ByteDance/Douyin official endpoints.
• Does not add a third-party parsing service or hard-coded credential.
• A configured deployment Cookie is sent only to the official Douyin detail endpoint and is never sent to the anonymous registration endpoint.

Fixes #98