feat(search): Add media format filter to advanced search.

[sreeshanth-soma]

Closes #170

Summary

Adds a Format filter to the media advanced search, allowing users to filter by file format (e.g., MP3, PDF, TIFF, OGG). The dropdown is dynamically populated with all enabled formats from Media Settings.

Changes

Backend

• core/models/media_model.php — Added format to $allowed_filters and mapped it to media.format in $column_array.

Frontend

• public/html/sidebar/advanced_search.html — Added "Format" option to the filter dropdown with a corresponding hidden <select> for format values.
• public/js/settings.js — Fetches enabled formats via media.formats_get API during getSettings(), stored as OB.Settings.formats.
• public/js/sidebar.js — Populates the Format dropdown by combining all enabled format types (audio, video, image, document) into a sorted, deduplicated list.