feat: add batch transcription entrypoints

[lanbinleo]

Summary

• add bili2text batch for submitting multiple transcription inputs from CLI args or a newline-separated file
• allow multiline source input in the Web UI and add a batch task status page
• add POST /api/tasks/batch for API batch submission
• allow multiline batch processing in the desktop window
• update docs, changelogs, and tests

Related Issue

Mentioning #82 because the issue discussion included a request for batch downloading / batch processing support.

Testing

• uv run pytest passed locally with 41 tests.
• This has not gone through formal end-to-end testing with real Bilibili downloads, because yt-dlp/Bilibili currently returns HTTP 412 upstream. The current validation covers parsing, task submission, Web/API logic, and fake-pipeline flows only.

Review Note

Please leave this PR open for review for a few days before merging.

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