Follow up video-library folder counts, gallery reuse, and thumbnail cleanup

[coderabbitai]

Summary

Track follow-up cleanup identified during video-support work in #1402.

Required changes

1. Extend db_get_all_folder_details to report video_count alongside the existing image_count.
2. Reduce duplication between ChronologicalVideoGallery and ChronologicalGallery by extracting a shared generic/gallery abstraction once the video UI has stabilized.
3. Remove thumbnail files when a folder is deleted. This applies to both image and video thumbnails; the image-side orphaning behavior predates the video feature.

Rationale

Folder details should accurately represent both supported media types, duplicated gallery logic will increase maintenance cost, and thumbnail cleanup prevents stale files from accumulating after folder deletion.

Affected areas

• Backend folder-detail database query/service (db_get_all_folder_details)
• Frontend chronological image/video gallery components
• Folder-deletion and thumbnail-storage cleanup logic for images and videos

Acceptance criteria

• Folder details expose a correct video_count without changing image_count semantics.
• The image and video chronological galleries share the common grouping/timeline behavior while retaining media-specific rendering.
• Deleting a folder removes its associated image and video thumbnail files, including safely handling missing files.
• Relevant backend and frontend tests cover the changes.

References

• Parent issue: Implement Videos Page with Video Support #1403
• Related pull request: Implement Videos Page with Video Support #1402
• Requesting comment: Implement Videos Page with Video Support #1402 (comment)
• Requested by: @rohan-pandeyy

[github-actions]

👋 Thanks for opening this issue! The maintainers will review it shortly. Till then, do not open any PR. This is a strict rule we like to follow around here :)

[sanskar-singh-2403]

hey @rohan-pandeyy can i pick this up?... if its fine.