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/types/podcast.ts

@@ -12,8 +12,12 @@ export interface Podcast {
   description: string
   /** Cover image URL */
   coverUrl?: string
-  /** RSS feed URL */
+  /** RSS feed URL. Empty when the directory lists the show without a feed
+   *  (e.g. shows delisted from Apple Podcasts); see directoryUrl. */
   feedUrl: string
+  /** Directory listing page (e.g. Apple Podcasts) for shows whose feed URL
+   *  the directory omits — used to resolve the real feed at subscribe time. */
+  directoryUrl?: string
   /** Author/creator name */
   author?: string
   /** Podcast categories */