* feat: add audio and video understanding via VLM

* docs: design media resource guards

* fix: bound media staging concurrency

* fix: cap unknown-size media staging

* test: stage media in routing fake

* test: exercise media staging callbacks

* test: trim media understanding coverage

* chore: 清理实现计划文档

* fix: 修复多凭证切换问题

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Co-authored-by: Qin Haojie <qinhaojie.exe@bytedance.com>