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/index.html

@@ -23,6 +23,7 @@
 			<button class="maint-toggle" id="maint-toggle" title="Maintenance">⚙</button>
 			<div class="maint-dropdown" id="maint-dropdown" hidden>
 				<button class="maint-item" id="maint-gen-thumbs">Generate missing PDF thumbnails</button>
+				<button class="maint-item" id="maint-purge-sources">Remove orphaned source image entries</button>
 			</div>
 		</div>
 	</header>