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Fix Douyin share video downloads failing when the default playback line resolves to an unavailable or rate-limited CDN.
- Keep a bounded alternate official playback line and preserve supplied video URL candidates, allowing the existing download flow to retry before sending media.
- Generate alternate lines only for recognized unsigned first-party share playback wrappers. Preserve signed CDN URLs and retain public-address checks on every redirect.
- Add regression coverage for alternate lines, opaque signed URLs, invalid wrappers, deduplication, and candidate limits.
Validation: full npm test suite passed; production dependency audit reported zero vulnerabilities. A real Douyin video completed through the Shortcut endpoint and passed full audio/video decoding. An isolated test with an injected primary-line 502 verified successful download through the alternate line. Production health reports 2.4.78.