feat(feed): bound feed lifecycle to 30-day window with nonblocking refresh

Persisted feeds keep only episodes from the last 30 days (plus completed
downloads); older episodes live in volatile memory and survive refreshes
via union merge, with per-feed in-memory caches capped at 500. Refresh
batches run at FETCH_CONCURRENCY=4 with per-feed incremental apply (no
Promise.all barrier), config.json writes are trailing-edge debounced
(250ms, immediate flushPendingSave for unsubscribes), and cold
fetch-more refetches abort at FETCH_TIMEOUT_MS. A shared activity store
powers a global top-right indicator covering refresh, fetch-more,
subscribe, search, and downloads.

Also includes the in-flight incremental RSS parsing (chunked with
event-loop yields) and refresh spinner work this tree already carried.

src/utils/config.ts

@@ -93,6 +93,13 @@ export function updateConfig(patch: Partial<PodTuiConfig>): void {
 	});
 }
 
+/** Resolve once every queued config write has flushed. Tests await this to
+ *  observe the serialized result of pending saveFeedsToFile/updateConfig
+ *  calls before asserting on config.json. */
+export function whenConfigIdle(): Promise<void> {
+	return writeChain;
+}
+
 /** Guards so migration runs exactly once per process. */
 let migrationDone = false;
 let migrationPromise: Promise<void> | null = null;