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();
 			}