feat(search): Add search by owner and group for media and playlists

[sreeshanth-soma]

Closes #133

Summary

Adds the ability to filter media and playlists by Owner (user) and Group in the advanced search interface.

Changes

Backend

• core/models/media_model.php — Added owner and group to allowed search filters. Owner maps to media.owner_id; group uses a subquery on media_permissions_groups.
• core/models/playlists_model.php — Added optional $owner and $group parameters to search(), filtering by owner_id and via subquery on playlists_permissions_groups.
• core/controllers/playlists.php — Forwards owner and group request data to the playlist model search.

Frontend

• public/html/sidebar/advanced_search.html — Added "Owner" and "Group" options to the filter dropdown, with corresponding hidden <select> elements for value selection.
• public/js/settings.js — Fetches user and group lists via users.user_list and users.group_list API calls during getSettings().
• public/js/sidebar.js — Populates the Owner and Group dropdowns from OB.Settings.users and OB.Settings.groups on advanced search init.