Add Files and Media field type

## Goal

A field that holds attachments per row. Multiple files per cell, uploaded through the WP media library. Useful for image-heavy collections, contracts, design references.

## Scope

* New field type, files, that stores attachment IDs in row meta.
* Upload via WP media library; pick existing attachments or upload new ones.
* Per-cell preview: thumbnails for images, filename for non-images.
* The gallery layout uses the field's first image as the card preview when configured.
* Remove and reorder files within a cell.

## Out of scope

* Inline file annotations.
* Versioning.
* Direct download permissions separate from row permissions.

## Acceptance criteria

* You can add a files field, attach one or more files to a row, and see thumbnails.
* You can remove and reorder files in a cell.
* Gallery cards can use the field's first image as the preview.
* Existing WP media library access controls apply.

The issue names no files or tests; start by locating the existing field-type implementations, row-meta storage, WordPress media-library integration, and gallery-card preview entry points. Done means users can attach, preview, remove, and reorder multiple files per row, and configured gallery cards use the first image while existing media access controls remain effective.

Written by the indexing model from the issue text.