Add bulk delete for videos

[yusufipk]

Summary

Allow project owners/admins to select and delete multiple videos at once from the project page, instead of deleting them one at a time from each video card.

Current behavior

• Videos can only be deleted individually via the per-card menu (VideoCard → Delete).
• The API supports single-video deletion only: DELETE /api/projects/[projectId]/videos/[videoId].
• Deleting many videos in a large project is slow and tedious.

Expected behavior

• Multi-select mode on the project video grid (checkboxes or similar).
• A bulk action bar with Delete selected (and optionally select all / clear selection).
• Confirmation dialog summarizing how many videos will be deleted and that versions, comments, and assets are included.
• Progress/feedback while bulk deletion runs; partial failures reported clearly.
• Reuse existing cleanup logic (Bunny stream, R2 media) per video.

Technical notes

• Existing single-delete handler in app/api/projects/[projectId]/videos/[videoId]/route.ts already handles Bunny/R2 cleanup — bulk endpoint or batched calls should share that logic.
• Project page UI lives in app/(dashboard)/projects/[projectId]/project-content-client.tsx with VideoCard components.
• Authorization: same as single delete — project owner or admin (checkProjectAccess with intent: 'manage').

Acceptance criteria

• User can enter selection mode and select multiple videos on the project page
• Bulk delete requires confirmation before proceeding
• Selected videos are removed from the UI after successful deletion
• Media cleanup (Bunny/R2) runs for each deleted video; warnings surfaced if cleanup partially fails
• Unauthorized users cannot bulk delete