feat: Sort Order for Series categories

[Warbs816]

Summary

Closes #1023.

Brings Series categories to parity with Live and VOD groups for sort ordering:

• Editor UI — adds the Sort Order column (inline-editable) and form field to the Series → Categories resource, mirroring the existing GroupResource / VodGroupResource layout. Drag-to-reorder already worked; this just exposes the value for direct editing.
• Xtream API output — get_series_categories on regular playlists / aliases now applies sortBy('sort_order'), so clients like TiVimate honor the custom order. Live and VOD already did this; series was the lone exception.

The categories.sort_order column itself already existed (default 9999), so no migration is needed. Existing auto_sort import behavior is untouched — it only ever applied to Live/VOD groups, and that hasn't changed.

Notes

• The tooltip on the Series category column reads Category sort order (no auto-sort warning), since auto_sort doesn't currently overwrite series categories during import.
• CustomPlaylist behavior is unchanged — that branch already sorted correctly.

Test plan

• Open Series → Categories: confirm new Sort Order column appears, edits persist, drag-reorder still works
• Edit a category: confirm Sort Order field is on the form and saves
• Hit player_api.php?action=get_series_categories on a regular playlist with reordered categories: confirm response order matches sort_order ASC
• Verify TiVimate (or another Xtream client) reflects the new category order