fix(web): pass end_user.external_user_id string to transcript_asr

[ki3nd]

AudioApi.post() was passing the raw EndUser object to AudioService.transcript_asr() which expects str | None. The object gets serialized as a dict when forwarded to the plugin daemon, causing Tongyi and other providers to fail with a type error.

Also adds EndUser type annotations to AudioApi and TextApi post methods.

Fixes #35877

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Summary

Screenshots

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Checklist

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran make lint && make type-check (backend) and cd web && pnpm exec vp staged (frontend) to appease the lint gods

[github-actions]

Pyrefly Type Coverage

Metric	Base	PR	Delta
Type coverage	0.00%	40.13%	+40.13%
Strict coverage	0.00%	39.65%	+39.65%
Typed symbols	0	20,088	+20,088
Untyped symbols	0	30,335	+30,335
Modules	0	2536	+2,536