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.

web/src/audio.ts

@@ -171,9 +171,20 @@ export function endTimeLabel(
 
 const PROGRESS_INTERVAL_MS = 30_000;
 
+interface PodcastSequenceConfig {
+  feed_url?: string;
+  feed_title?: string;
+  prev_url?: string;
+  next_url?: string;
+  next_title?: string;
+  episode_index?: number;
+  index_total?: number;
+}
+
 interface AudioConfig {
   chapterEndpoint: string;
   progressEndpoint: string;
+  podcastSequence?: PodcastSequenceConfig;
 }
 
 function readConfig(): AudioConfig {
@@ -419,10 +430,14 @@ Alpine.data("audioReader", () => ({
     });
     audio.addEventListener("ended", () => {
       // Natural end of track n: roll into n+1 and keep playing. At the
-      // last track the book is done — nothing to advance into.
+      // last track the book is done — nothing to advance into... unless
+      // this is a podcast episode with a sequence (D7): chain into the
+      // next downloaded episode's reader page.
       if (this.activeTrack < this.tracks.length - 1) {
         this.setActiveTrack(this.activeTrack + 1, 0, true);
+        return;
       }
+      this.goToNextEpisode(true);
     });
     audio.addEventListener("play", () => {
       this.playing = true;
@@ -760,9 +775,42 @@ Alpine.data("audioReader", () => ({
 
   backToDetail() {
     this.pushProgress(true);
+    // Podcast episodes exit to their feed page (D7 sequence context);
+    // books keep the book-detail exit.
+    const seq = readConfig().podcastSequence;
+    if (seq && seq.feed_url) {
+      window.location.href = seq.feed_url;
+      return;
+    }
     window.location.href = `/media/${this.mediaItemId}`;
   },
 
+  // ---- podcast sequence chaining (D7) ----
+  // Auto-advance across episodes: the last track's `ended` rolls into
+  // the next episode's reader page (progress already pushed on
+  // timeupdate; a final push fires before navigating). Books are
+  // untouched — no sequence context, no behavior change.
+  goToNextEpisode(finalPush) {
+    const seq = readConfig().podcastSequence;
+    if (!seq || !seq.next_url) return false;
+    const go = () => {
+      window.location.href = seq.next_url as string;
+    };
+    if (finalPush) {
+      Promise.resolve(this.pushProgress(true)).then(go).catch(go);
+    } else {
+      go();
+    }
+    return true;
+  },
+  goToPrevEpisode() {
+    const seq = readConfig().podcastSequence;
+    if (seq && seq.prev_url) {
+      this.pushProgress(true);
+      window.location.href = seq.prev_url;
+    }
+  },
+
   // ---- volume (settings strip) ----
 
   volumeIcon(): "muted" | "low" | "mid" | "high" {