perf(frontend): parallelize video thumbs, batch owner-cache writes, minor tweaks

- photos: generate + upload the three video-thumbnail sizes in parallel via
  Promise.allSettled instead of a sequential await loop; previewData is still
  captured before its upload so the local preview survives a failed upload.
- useOwnerCache: resolve the batch, then apply a single reactive assignment
  to `#names` instead of spread-copying the record once per id (fewer copies
  and fewer derive re-runs on large resolves).
- admin: migration status polling slowed from 2s to 5s.
- files store: soft cap (10k) on the per-item selection toggle. Bulk
  "select all" in the views is intentionally left uncapped.

frontend/src/routes/admin/+page.svelte

@@ -320,7 +320,7 @@
 		try {
 			migration = await getMigration();
 			if (migration.status === 'running') {
-				if (!migrationTimer) migrationTimer = setInterval(loadMigration, 2000);
+				if (!migrationTimer) migrationTimer = setInterval(loadMigration, 5000);
 			} else {
 				stopMigrationPoll();
 			}

frontend/src/routes/photos/+page.svelte

@@ -301,11 +301,16 @@
 				['large', 800, 800]
 			];
 			let previewData = '';
-			for (const [size, w, h] of SIZES) {
-				const blob = await bitmapToBlob(bitmap, w, h);
-				if (size === 'preview') previewData = await blobToDataUrl(blob);
-				await uploadThumbnail(file.id, size, blob).catch(() => {});
-			}
+			// Render the blobs and push all three sizes in parallel; `previewData`
+			// is captured before its upload so the local preview shows even if that
+			// upload fails (allSettled swallows per-size failures, as before).
+			await Promise.allSettled(
+				SIZES.map(async ([size, w, h]) => {
+					const blob = await bitmapToBlob(bitmap, w, h);
+					if (size === 'preview') previewData = await blobToDataUrl(blob);
+					await uploadThumbnail(file.id, size, blob);
+				})
+			);
 			if (previewData) videoThumbs = { ...videoThumbs, [file.id]: previewData };
 		} catch {
 			// Keep the generic play badge on failure.