Add filter by Alt text content to the Media Alts view

Problem/Motivation

The Media Alts view (/admin/content/media-alts) provides a convenient interface to review and manage alt text attributes across media items. However, there is currently no way to filter results by the content of the alt attribute itself.

Use cases wher this filter might be useful:

• Removing test or debug alt text: strings like "test", "TODO", "lorem", or "asdf" that were entered during development or content import and were never replaced with proper descriptions before going to production
• Detecting duplicated alt text: Searches for a repeated alt string that was copy-pasted across many images.
• Multilingual content review: — A site managing multiple languages needs to filter media by alt text in a specific language to verify translations are in place and accurate (Ideally should be combined with language filter).

Proposed resolution

Add an exposed filter on the alt text field (or the underlying image field alt column) to the Media Alts view, so administrators can narrow down the list by alt attribute content.

Remaining tasks

User interface changes

API changes

Data model changes