Add maintenance: purge orphaned source image KV entries

When a source image file is deleted without going through the API
(e.g. the old bin delete bug), the KV entry remains and shows a
broken image. The new maintenance action scans all source image
entries, removes any whose file is missing on disk, and reports
how many were cleaned up.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

public/lib/api.mjs

@@ -86,7 +86,8 @@ export async function upload_bin(file, name) {
 }
 
 // Maintenance
-export const maintenance_pdf_thumbs = () => req('POST', '/api/maintenance/pdf-thumbs');
+export const maintenance_pdf_thumbs        = () => req('POST', '/api/maintenance/pdf-thumbs');
+export const maintenance_purge_missing_sources = () => req('POST', '/api/maintenance/purge-missing-sources');
 
 // Grid images
 export const get_grids        = () => req('GET', '/api/grid-images');