Describe the featured image without opening the media library

[ssavutu]

Closes the last gap in in-place alt-text editing. Body images and library records both got it in #190; the featured image did not.

What was wrong

The Featured Image block showed a thumbnail, Change image and Remove. It never displayed the alt text, never warned when there was none, and offered no way to set one. The only route was to click Change image, find the row in the picker, edit the alt there, and close without changing the image — a destructive-looking flow to make a non-destructive edit.

The alt was also discarded on the way in: onSelect kept item.url and dropped item.alt_text, so an image described once in the library arrived at the featured slot undescribed.

Why article-scoped

New photo_alt column rather than a lookup into media.alt_text:

• The same photo can want a different description in a different story — the same argument that made body attachments article-scoped.
• A featured image set via Use URL has no library record to read one from.
• The featured image is stored as a bare URL, so a library-scoped edit would have to resolve that URL back to a row before it knew what to patch.

Nothing is lost by not reading the library. Scalene currently renders alt={normalizeText(post.title)} — the headline, repeated next to itself, which tells a screen-reader user nothing new. The public site can consume featured_image_alt in its own change; until then the CMS is the only place the real description exists.

Behaviour

• Alt text is editable inline in the Featured Image block and saves with the article.
• Picking an image adopts the library's description, but only when the image actually changes, so re-picking the same photo cannot overwrite a description written here.
• Removing the image clears the alt — it belonged to the photo being removed.
• Missing alt is called out in the destructive colour rather than left silent.

Testing

• TestArticlePatchHTTP_PhotoAltRoundTrip — persists trimmed, clears on empty, rejects a non-string with 400. Run against a real MariaDB, not just skipped.
• autosaves featured image alt text on its own — the field is in the autosave snapshot. This has been the failure mode twice: a field outside the body editor that is missing from the snapshot never marks the article dirty and the edit is silently discarded.
• Full go test ./... with CMS_TEST_DSN set, tsc --noEmit, vitest run, eslint all clean. The one remaining lint warning (saveArticle dep) is pre-existing on main.
• Verified the EnsureArticlesSchema ALTER applies to a table that predates the column.

Notes

The hint text sits outside its <label> with aria-describedby. Inside, it would have been read as part of the field's name rather than its description.

Adding an article column means touching four SELECT lists feeding one positional ScanArticle, plus the two integration-test schemas that build their own articles table — the full DB-backed suite is what caught those.

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