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/audio_reader.templ

@@ -7,7 +7,32 @@ package templates
 // <audio> element streams the m4b through the existing cookie-authed
 // Range-serving /uploads path.
 
-import "fmt"
+import (
+	"encoding/json"
+	"fmt"
+)
+
+// AudioReaderConfigJSON builds the reader's body[data-json] config: the
+// chapter/progress endpoints plus, for podcast episodes, the sequence
+// context (chain playback — plan D7). Books render the same JSON shape
+// WITHOUT the sequence key, so the reader code path is identical.
+func AudioReaderConfigJSON(metadata ReaderMetadata) string {
+	cfg := map[string]any{
+		"chapterEndpoint":  "/api/media-items/" + metadata.MediaItemID + "/chapters",
+		"progressEndpoint": "/api/media-items/" + metadata.MediaItemID + "/progress",
+	}
+	if metadata.PodcastSequence != nil {
+		cfg["podcastSequence"] = metadata.PodcastSequence
+	}
+	b, err := json.Marshal(cfg)
+	if err != nil {
+		// Unreachable for these shapes; degrade to the book-only config.
+		return fmt.Sprintf(`{"chapterEndpoint":"%s","progressEndpoint":"%s"}`,
+			"/api/media-items/"+metadata.MediaItemID+"/chapters",
+			"/api/media-items/"+metadata.MediaItemID+"/progress")
+	}
+	return string(b)
+}
 
 templ AudioReader(user User, metadata ReaderMetadata) {
 	<!DOCTYPE html>
@@ -41,9 +66,7 @@ templ AudioReader(user User, metadata ReaderMetadata) {
 			data-cover={ metadata.CoverImagePath }
 			data-file-url={ metadata.FileURL }
 			data-keep-awake={ metadata.KeepScreenAwake }
-			data-json={ fmt.Sprintf(`{"chapterEndpoint":"%s","progressEndpoint":"%s"}`,
-				"/api/media-items/"+metadata.MediaItemID+"/chapters",
-				"/api/media-items/"+metadata.MediaItemID+"/progress") }
+			data-json={ AudioReaderConfigJSON(metadata) }
 		>
 			<!-- §17 wind-down dim: the sleep ticker eases this toward 35%
 			     black across a countdown's final 30s; pointer-events-none
@@ -55,18 +78,37 @@ templ AudioReader(user User, metadata ReaderMetadata) {
 			<!-- Header: back to the detail page (reader-style exit) and the
 			     chapter launcher. Floating drawer, never in-flow (A2b). -->
 			<div class="absolute top-0 inset-x-0 z-30 flex items-center justify-between px-4 py-3">
-				<button
-					type="button"
-					@click="backToDetail()"
-					class="rounded-full p-2 hover:bg-surface-hover transition-colors"
-					style="color: var(--text-primary);"
-					aria-label="Back to book details"
-				>
-					<svg width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true">
-						<path d="M19 12H5"></path>
-						<path d="m12 19-7-7 7-7"></path>
-					</svg>
-				</button>
+				<span class="inline-flex items-center gap-1">
+					<button
+						type="button"
+						@click="backToDetail()"
+						class="rounded-full p-2 hover:bg-surface-hover transition-colors"
+						style="color: var(--text-primary);"
+						aria-label="Back to book details"
+					>
+						<svg width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true">
+							<path d="M19 12H5"></path>
+							<path d="m12 19-7-7 7-7"></path>
+						</svg>
+					</button>
+					<!-- Podcast sequence (D7): episode prev/next + "Ep N of M".
+					     Rendered only when the server supplied sequence context. -->
+					if metadata.PodcastSequence != nil {
+						<span class="text-xs tabular-nums ml-1" style="color: var(--text-secondary)">
+							Ep { fmt.Sprintf("%d", metadata.PodcastSequence.EpisodeIndex) }/{ fmt.Sprintf("%d", metadata.PodcastSequence.IndexTotal) }
+						</span>
+						if metadata.PodcastSequence.PrevURL != "" {
+							<a href={ metadata.PodcastSequence.PrevURL } class="rounded-full p-2 hover:bg-surface-hover transition-colors" style="color: var(--text-primary);" aria-label="Previous episode" title="Previous episode">
+								<svg width="18" height="18" viewBox="0 0 24 24" fill="currentColor" aria-hidden="true"><path d="M6 5h2v14H6zM20 5v14l-11-7z"></path></svg>
+							</a>
+						}
+						if metadata.PodcastSequence.NextURL != "" {
+							<a href={ metadata.PodcastSequence.NextURL } class="rounded-full p-2 hover:bg-surface-hover transition-colors" style="color: var(--text-primary);" aria-label={ "Next episode: " + metadata.PodcastSequence.NextTitle } title={ "Next: " + metadata.PodcastSequence.NextTitle }>
+								<svg width="18" height="18" viewBox="0 0 24 24" fill="currentColor" aria-hidden="true"><path d="M16 5h2v14h-2zM4 5v14l11-7z"></path></svg>
+							</a>
+						}
+					}
+				</span>
 				<span class="inline-flex items-center gap-2">
 					<button
 						type="button"