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.

internal/router/reader.go

@@ -1,13 +1,17 @@
 package router
 
 import (
+	"bookhoard/internal/database"
 	"bookhoard/internal/handlers"
 	"bookhoard/internal/services"
 	"bookhoard/internal/sync"
 	"bookhoard/internal/utils"
 	"bookhoard/templates"
 	"bytes"
+	"encoding/json"
+	"fmt"
 	"net/http"
+	"sort"
 
 	"github.com/google/uuid"
 	"github.com/jackc/pgx/v5/pgtype"
@@ -102,6 +106,18 @@ func registerReaderRoutes(cfg *Config) {
 			TreatAsComic:     handlers.ShouldTreatAsComic(libraryWithType.TypeName, mediaItem.FormatGroup),
 			KeepScreenAwake:  user.KeepScreenAwake,
 		}
+		// Podcast chaining (plan D7): episodes render the audio reader
+		// with sequence context — the neighboring DOWNLOADED episodes in
+		// the caller's effective order (override applied). RLS already
+		// scoped the episode read; the subscription predicate covers the
+		// neighbors the same way. ?single=1 escapes chaining (the "play
+		// single episode" affordance).
+		if mediaItem.Source == "podcast_episode" && mediaItem.SourceFeedID.Valid && c.QueryParam("single") != "1" {
+			userPG := uuidToPGType(userUUID)
+			if seq := buildPodcastSequenceContext(c, cfg, &mediaItem, userPG, libraryWithType.TypeName); seq != nil {
+				metadata.PodcastSequence = seq
+			}
+		}
 		// Render template
 		var buf bytes.Buffer
 		// Audiobooks get the audio player page — the foliate document
@@ -144,3 +160,96 @@ func registerReaderRoutes(cfg *Config) {
 	reader.GET("/settings", cfg.ReaderHandler.GetSettings)
 	reader.PUT("/settings", cfg.ReaderHandler.UpdateSettings)
 }
+
+// buildPodcastSequenceContext computes the episode chain payload (plan
+// D7): effective order — the D5 override applied, feed-native fallback —
+// DOWNLOADED episodes only (gaps never enter the queue), the current
+// episode's index, and prev/next reader URLs. RLS scopes the episode
+// reads; non-subscribers never reach this path in the first place.
+func buildPodcastSequenceContext(c *echo.Context, cfg *Config, item *database.MediaItems, userID pgtype.UUID, libraryType string) *templates.PodcastSequenceContext {
+	ctx := c.Request().Context()
+	eps, err := cfg.Queries.ListPodcastEpisodes(ctx, item.SourceFeedID)
+	if err != nil || len(eps) == 0 {
+		return nil
+	}
+	feed, err := cfg.Queries.GetPodcastFeed(ctx, item.SourceFeedID)
+	if err != nil {
+		return nil
+	}
+
+	// Effective order override (D5): reader_settings KV.
+	order := "native"
+	if row, err := cfg.Queries.GetUserSettingByKey(ctx, database.GetUserSettingByKeyParams{
+		UserID:     userID,
+		SettingKey: "podcast_order:" + uuid.UUID(item.SourceFeedID.Bytes).String(),
+	}); err == nil {
+		var v struct {
+			Order string `json:"order"`
+		}
+		if json.Unmarshal(row, &v) == nil && (v.Order == "oldest" || v.Order == "newest") {
+			order = v.Order
+		}
+	}
+
+	type seqEp struct {
+		id    string
+		title string
+		date  string
+		pos   int32
+	}
+	ordered := make([]seqEp, 0, len(eps))
+	currentID := uuid.UUID(item.ID.Bytes).String()
+	for i := range eps {
+		ep := eps[i]
+		if ep.DownloadState != "downloaded" {
+			continue // un-downloaded episodes are skipped, not tracked (invariant 6)
+		}
+		date := ""
+		if ep.DatePublished.Valid {
+			date = ep.DatePublished.Time.Format("2006-01-02")
+		}
+		ordered = append(ordered, seqEp{
+			id:    uuid.UUID(ep.ID.Bytes).String(),
+			title: ep.Title,
+			date:  date,
+			pos:   ep.FeedPosition.Int32,
+		})
+	}
+	sort.SliceStable(ordered, func(a, b int) bool {
+		ka := ordered[a].date + fmt.Sprintf("%06d", ordered[a].pos)
+		kb := ordered[b].date + fmt.Sprintf("%06d", ordered[b].pos)
+		if order == "newest" {
+			return ka > kb
+		}
+		if order == "oldest" {
+			return ka < kb
+		}
+		return ordered[a].pos < ordered[b].pos
+	})
+
+	idx := -1
+	for i, ep := range ordered {
+		if ep.id == currentID {
+			idx = i
+			break
+		}
+	}
+	if idx < 0 {
+		return nil // current episode not downloaded (or not in sequence)
+	}
+
+	seq := &templates.PodcastSequenceContext{
+		FeedURL:      "/podcasts/" + uuid.UUID(item.SourceFeedID.Bytes).String(),
+		FeedTitle:    feed.Title,
+		EpisodeIndex: idx + 1,
+		IndexTotal:   len(ordered),
+	}
+	if idx > 0 {
+		seq.PrevURL = "/readers/" + ordered[idx-1].id
+	}
+	if idx < len(ordered)-1 {
+		seq.NextURL = "/readers/" + ordered[idx+1].id
+		seq.NextTitle = ordered[idx+1].title
+	}
+	return seq
+}