feat(video): JianYing Pro (剪映) draft project export for NLE post-editing

Problem

OpenMontage provides automated pipelines for AI video generation (scripting, TTS narration, scene/visual synthesis, BGM, and composition). However, creators often require final human-in-the-loop review, timing micro-adjustments, or creative polish in a desktop NLE (Non-Linear Editor) before publishing.

In the Chinese content creator ecosystem, JianYing Pro (剪映专业版) is the predominant desktop editing suite. Currently, OpenMontage has no native export integration for JianYing project drafts. Creators must manually import raw media files, recreate tracks, re-align audio, and re-import subtitle files.

Proposed Solution

Add a dedicated jianying_draft tool in tools/video/jianying_draft.py:

• Assemble timeline elements (video/image clips, transitions, voiceovers, BGM, and SRT subtitles) directly into native JianYing draft projects using pyJianYingDraft.
• Support automatic registration into JianYing's root_meta_info.json so generated drafts immediately appear on JianYing Pro's home screen under "Recent Projects".
• Provide graceful degradation if pyJianYingDraft is not installed or when running on non-Windows environments.
• Provide a CLI entrypoint for batch / standalone usage (python -m tools.video.jianying_draft ...).

Happy to submit a clean PR with unit and contract tests.