fix(search): surface iTunes stubs and resolve feeds for delisted shows

Shows that left Apple Podcasts (e.g. Daily Wire's in 2021) come back from
the iTunes Search API as metadata-only stub records with feedUrl null.
mapItunesResult dropped them, so The Ben Shapiro Show — the #1 hit for
'ben shapiro' — never appeared in search while sibling shows did.

- Keep feedUrl-less results (feedUrl "" + directoryUrl pointing at the
  Apple page) so delisted shows stay findable.
- Resolve the real feed from the Apple page at subscribe time
  (itunes-feed-resolver: anchor on the collection's adamId, forward-scan
  for the embedded feedUrl; Apple serves page variants where the
  showOffer block sits thousands of chars after the adamId).
- addFeed refuses feedless stubs whose feed can't be resolved instead of
  adding a broken feed; SearchPage surfaces the failure via toast.
- Tests: stub mapping, extractor variants, and an end-to-end subscribe
  over a local HTTP server.

src/stores/feed.ts

@@ -11,6 +11,7 @@ import type { Episode } from "../types/episode";
 import type { PodcastSource } from "../types/source";
 import { DEFAULT_SOURCES } from "../types/source";
 import { parseRSSFeed } from "../api/rss-parser";
+import { resolveItunesFeedUrl } from "../utils/itunes-feed-resolver";
 import {
 	loadFeedsFromFile,
 	saveFeedsToFile,
@@ -196,6 +197,17 @@ function createFeedStore() {
 		sourceId: string,
 		visibility: FeedVisibility = FeedVisibility.PUBLIC,
 	): Promise<Feed | null> => {
+		// A directory stub (e.g. a show delisted from Apple Podcasts) has no
+		// feed URL; resolve the real feed from its directory page before
+		// subscribing. Refuse when it can't be resolved rather than adding a
+		// broken feed.
+		if (!podcast.feedUrl) {
+			if (!podcast.directoryUrl) return null;
+			const resolved = await resolveItunesFeedUrl(podcast.directoryUrl);
+			if (!resolved) return null;
+			podcast = { ...podcast, feedUrl: resolved, directoryUrl: undefined };
+		}
+
 		// Guard: don't add a feed we already have (matched by feedUrl)
 		if (hasFeedByUrl(podcast.feedUrl)) {
 			return feeds().find((f) => f.podcast.feedUrl === podcast.feedUrl) ?? null;