feat(podcasts): chain playback + Continue section — sequence mode in the web player (phase 2 core)

D7 chain-by-default, delivered with minimal player surgery:

- ReaderMetadata gains PodcastSequence: the neighboring DOWNLOADED
  episodes in the caller's effective order (D5 override applied,
  feed-native fallback; computed in the reader route for
  source='podcast_episode' items — RLS already scoped the reads; gaps
  never enter the chain, invariant 6). ?single=1 escapes chaining (the
  'play single episode' affordance).
- Audio reader renders the sequence into body[data-json]
  (AudioReaderConfigJSON — books produce the identical shape minus the
  sequence key, so the reader path is unchanged for them) and an
  'Ep N/M' indicator + prev/next episode controls in the header.
- audio.ts: the last-track 'ended' handler chains into the next
  episode's reader page (final progress push precedes navigation);
  back button exits episodes to their feed page. Books are untouched —
  no sequence context, zero behavior change. Day-one chaining is
  page-navigation based (progress persists; position restores).

Continue aggregation (2.5): GetPodcastContinueForUser (in-progress
episodes, most-recent-first, ≥95% excluded) feeds a Continue section
on /podcasts — sorted most recently listened, cards show 'Ep N · title'
— ahead of All Feeds. Cards prefer the continue line over the
description.

templates/podcast_detail.templ

@@ -270,7 +270,7 @@ templ PodcastEpisodeRowView(ep PodcastEpisodeRow) {
 			</p>
 		</div>
 		if ep.Downloaded {
-			<a href={ ep.PlayURL } class="btn btn-secondary shrink-0" title="Play episode">
+			<a href={ ep.PlayURL } class="btn btn-secondary shrink-0" title="Play — chains into the next downloaded episode (append ?single=1 to the URL to play just this one)">
 				@Icon("book-open", "h-4 w-4")
 				<span>Play</span>
 			</a>