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/app.mjs

@@ -2509,6 +2509,25 @@ async function init() {
 		}
 	});
 
+	document.getElementById('maint-purge-sources').addEventListener('click', async () => {
+		maint_dropdown.hidden = true;
+		maint_toggle.textContent = '⏳';
+		maint_toggle.disabled = true;
+		try {
+			const result = await api.maintenance_purge_missing_sources();
+			if (result.removed.length > 0) {
+				all_sources = all_sources.filter(s => !result.removed.includes(s.id));
+				render();
+			}
+			alert(`Removed ${result.removed.length} orphaned entr${result.removed.length === 1 ? 'y' : 'ies'} of ${result.total} checked.`);
+		} catch (err) {
+			alert(`Error: ${err.message}`);
+		} finally {
+			maint_toggle.textContent = '⚙';
+			maint_toggle.disabled = false;
+		}
+	});
+
 	window.addEventListener('popstate', () => { parse_url(); render(); });
 
 	await load_all();