Add intro and outro clips to the note video export

[davior]

What

The Generate video dialog gets a new Intro & outro tab. You can upload one clip to play before the title screen and another to play after the last section.

• Each clip plays whole, with its own sound. A clip with no audio track gets silence. It is fitted to the frame the same way as any video in a note, so a 4:3 or 9:16 clip still works in a 16:9 render.
• Nothing of the render is drawn over a clip: no watermark, overlay text or waveform. In "title + chapter" overlay mode, the chapter line never shows "Intro".
• The background music starts after the intro and stops before the outro. Fades happen at those edges, and ducking still works.
• Transitions still join the clips to the rest of the video (cuts, dips and crossfades).
• Each clip gets an Intro / Outro chapter marker in the MP4.
• The poster frame comes from the article, not the intro.
• The dialog's length estimate includes the clips.
• If a clip is missing, isn't a video, or belongs to another user, it is skipped with a warning on the job instead of failing the render.

Persistence / storage

• Clips are uploaded into the user's own media folder (/media/{user_id}/…), not into the note. They're meant to be reused on every video.
• The dialog's options are now saved to the account as the video_render_options user setting (debounced 800 ms, and flushed when the dialog closes), as well as to localStorage. They now follow the user to other browsers. The account copy wins when it exists; otherwise the dialog falls back to the browser's older local copy. Just opening the dialog never writes, so a stale local copy can't overwrite what another device saved.
• Files a saved setting points at count as in use. settings_media_filenames() feeds both file_is_referenced() and the unlinked-file sweep. Without this, the unlinked-file sweep would have offered the intro and outro (and the existing watermark icon and music track, which already had this problem) for deletion. Once a clip is removed in the dialog, the sweep offers it again.

Testing

• Backend: pytest gives 1138 passed (22 new):
  • segmenter: the clips wrap the article; switching one off, missing files, other users' files and still images are all handled; an empty note stays empty
  • music-window filtergraph
  • full render() through a fake ffmpeg (test_video_bumpers.py): no overlays or waveform on a clip, music window, chapters, poster time, crossfade timing
  • settings-referenced files are kept by the unlinked sweep and by asset deletes
• Real ffmpeg render with a 4:3 intro that has sound and a 9:16 outro with no audio track, under cut, dip and dissolve. All three produce valid MP4s with Intro/Outro chapters. Measured audio shows no music under either clip. The extracted frames show the clips fitted with nothing drawn over them, while the article keeps its watermark, overlay and waveform.
• Frontend: tsc --noEmit, vitest (190 passed), npm run build.
• Browser (Playwright, Vite + uvicorn): uploaded an intro and checked it was saved to /api/settings with its URL and filename. After a reload with the local copy cleared, the dialog still showed it. The unlinked scan didn't list it while it was in use and did list it after Remove. No page errors.

🤖 Generated with Claude Code

https://claude.ai/code/session_01UbALFUcK5VxpopwohSkG7N

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Generated by Claude Code