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/lib/composables/useOwnerCache.svelte.ts

@@ -31,14 +31,16 @@ export class OwnerCache {
 
 	/** Resolve every not-yet-cached id in parallel; nullish ids are skipped. */
 	async resolve(ids: Iterable<string | null | undefined>): Promise<void> {
-		const unique = [...new Set([...ids].filter((id): id is string => !!id))];
-		await Promise.all(
-			unique.map(async (id) => {
-				if (this.#names[id]) return;
-				const name = await this.#resolver(id);
-				this.#names = { ...this.#names, [id]: name };
-			})
+		const pending = [...new Set([...ids].filter((id): id is string => !!id))].filter(
+			(id) => !this.#names[id]
 		);
+		if (pending.length === 0) return;
+		const resolved = await Promise.all(
+			pending.map(async (id) => [id, await this.#resolver(id)] as const)
+		);
+		// One reactive assignment for the whole batch instead of one per id, so a
+		// large resolve doesn't spread-copy the record N times (and re-run derives N times).
+		this.#names = { ...this.#names, ...Object.fromEntries(resolved) };
 	}
 }