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/database/querier.go

@@ -455,6 +455,11 @@ type Querier interface {
 	// Get OPDS tokens by device
 	GetOpdsTokensByDevice(ctx context.Context, deviceID pgtype.UUID) ([]OpdsTokens, error)
 	GetPanelData(ctx context.Context, arg GetPanelDataParams) (PanelData, error)
+	// Continue aggregation (plan 2.5): the caller's IN-PROGRESS episodes
+	// (0 < pct < finishedThreshold) across ALL subscribed feeds, most
+	// recent first. Go groups per feed — the first row per feed IS the
+	// continue target.
+	GetPodcastContinueForUser(ctx context.Context, userID pgtype.UUID) ([]GetPodcastContinueForUserRow, error)
 	GetPodcastEpisode(ctx context.Context, id pgtype.UUID) (MediaItems, error)
 	// Per-user progress for a feed's episodes (row badges + the Phase-2
 	// continue fields). reading_progress is UNIQUE per (media_item, user).