Require a Description (alt text) when uploading images

[brylie]

Templates use Wagtail's image Description field as alt text, falling back to the title. Editors can still save an image with no description, which leaves screen-reader users with a filename-like alt text (WCAG 1.1.1). Make the Description required when an image is uploaded or edited.

Grounding: docs/accessibility-audit-2026-09.md, "Remaining recommendations (not fixed in code)", item 3

Acceptance criteria:

• The image upload and edit forms require a Description (a custom image form, or Wagtail's WAGTAILIMAGES_IMAGE_FORM_BASE setting)
• Help text tells editors to describe what the image shows
• Existing images without a Description are unaffected, or a management command lists them
• Tests cover the form validation