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/queries/queries.sql

@@ -5009,3 +5009,22 @@ JOIN media_items mi ON mi.id = rp.media_item_id
 WHERE rp.user_id = $1
   AND mi.source_feed_id = $2
   AND mi.source = 'podcast_episode';
+
+-- name: GetPodcastContinueForUser :many
+-- 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.
+SELECT mi.source_feed_id,
+       mi.id,
+       mi.title,
+       mi.feed_position,
+       rp.percentage,
+       rp.last_read_at
+FROM reading_progress rp
+JOIN media_items mi ON mi.id = rp.media_item_id
+WHERE rp.user_id = $1
+  AND mi.source = 'podcast_episode'
+  AND rp.percentage > 0
+  AND rp.percentage < 0.995
+ORDER BY rp.last_read_at DESC;