feat(podcasts): episode collections — picker scope, hint rows, the chain (item 2.3)

D11 lifts: podcast episodes (media_items rows) join collections, and
"Play collection as sequence" feeds the player from the collection's
order. No new tables — collection_items already carries ordered
membership (item 2.1 audit).

PICKER: the collection modal gains a Books/Episodes tab pair. The
episodes tab hits /api/media-items/search?scope=episodes — a dedicated
picker query (SearchEpisodesForPicker) whose visibility is entirely
RLS: the media_items subscription predicate drops unsubscribed feeds'
episodes, and the podcast_feeds join is governed the same way. The
books scope keeps its podcasts exclusion (plan §9) — this scope is the
only door in. Pending episodes are pickable (plan ahead; the detail
page shows them as not downloaded). Selection rides the existing
bookPicker store — ids never collide across scopes, and AddBooks works
unchanged on episode ids.

DETAIL: the All-Libraries membership view switches to
GetCollectionItemsDetailed — LEFT JOIN + per-row visibility. An
unsubscribed episode (or family-lens hidden book) survives as a
Hidden card rendered as an "Unavailable — remove entry" hint row
instead of silently vanishing; visible episodes render feed title,
download state, and a Play link. The library-scoped view keeps its
existing dashboard-query behavior (the switcher's contract is
untouched for books).

CHAIN: ?queue=collection:<id> on any reader URL makes
buildPodcastSequenceContext produce the COLLECTION context (§10 R2
same shape, source=collection): downloaded episodes in collection
order (positions are content — R1; no feed-order override), prev/next
URLs carrying the queue param so the chain survives hops, and
backToDetail landing on the collection page. Ownership gates the chain
— foreign/deleted ids fall back to the feed queue; the order control
stays hidden (2.2's source gate). The header's "Play as Sequence"
button starts from the first downloaded episode in list order.

Pinned by TestPodcastEpisodeCollections (7 subtests): RLS-scoped
picker + family-lens auditor case (subscribed episode in a hidden
library must not reach the picker), books-scope exclusion intact,
hint rows, chain carrying the queue param, start-from-first, foreign
fallback, and reorder driving the chain (the R4 primitive works on
episode ids unchanged).

internal/database/queries/queries.sql

@@ -932,6 +932,26 @@ ORDER BY
   mi.title ASC
 LIMIT sqlc.narg('limit') OFFSET sqlc.narg('offset');
 
+-- name: SearchEpisodesForPicker :many
+-- Episode picker scope (plan item 2.3): YOUR subscriptions' episodes.
+-- The podcasts exclusion in SearchMediaItemsUnified stays (books
+-- surfaces never list podcasts — plan §9); THIS query is the episode
+-- picker's dedicated scope. Visibility is the engine's job (D20): the
+-- media_items policy drops episodes from unsubscribed feeds and the
+-- podcast_feeds join is governed the same way — non-subscribers see
+-- nothing here without any Go-side clause.
+SELECT mi.id, mi.title, mi.author, mi.cover_image_path, mi.library_id,
+       mi.format_group, mi.download_state,
+       pf.title AS feed_title
+FROM media_items mi
+JOIN podcast_feeds pf ON mi.source_feed_id = pf.id
+WHERE mi.source = 'podcast_episode'
+  AND mi.archived_at IS NULL
+  AND mi.missing_scan_count = 0
+  AND (sqlc.arg('q')::text = '' OR mi.title ILIKE '%' || sqlc.arg('q')::text || '%')
+ORDER BY pf.title ASC, mi.feed_position ASC NULLS LAST
+LIMIT 60;
+
 -- name: SearchAuthorValues :many
 SELECT
   mi.author as value,
@@ -3576,6 +3596,42 @@ WHERE ci.collection_id = $1
   AND mi.missing_scan_count = 0
 ORDER BY ci.position ASC NULLS LAST, ci.added_at ASC;
 
+-- name: GetCollectionItemsDetailed :many
+-- Episode-collections detail (plan item 2.3): the collection's full
+-- membership for the SSR detail page, with per-row visibility. LEFT
+-- JOIN + RLS: rows whose media is invisible to the requester
+-- (unsubscribed episode — or a family-lens hidden book) survive with
+-- NULL media columns and visible=false, so the page can render an
+-- "unavailable" hint instead of silently vanishing them. Rows visible
+-- but archived/missing-scan keep the old INNER-JOIN behavior (dropped).
+SELECT ci.media_item_id, ci.position, ci.added_at,
+       (mi.id IS NOT NULL)::bool AS visible,
+       mi.title, mi.author, mi.cover_image_path, mi.format_group,
+       mi.source, mi.download_state,
+       pf.title AS feed_title
+FROM collection_items ci
+LEFT JOIN media_items mi ON ci.media_item_id = mi.id
+LEFT JOIN podcast_feeds pf ON mi.source_feed_id = pf.id
+WHERE ci.collection_id = $1
+  AND (mi.id IS NULL OR (mi.archived_at IS NULL AND mi.missing_scan_count = 0))
+ORDER BY ci.position ASC NULLS LAST, ci.added_at ASC;
+
+-- name: GetCollectionEpisodesOrdered :many
+-- The collection queue's downloaded episodes in collection order
+-- (plan item 2.3 "Play collection as sequence"): position-first, then
+-- added_at. INNER JOIN — the chain steps only through episodes the
+-- requester can see (RLS drops unsubscribed feeds) and that are
+-- downloaded (invariant 6: un-downloaded episodes are never tracked).
+SELECT mi.id, mi.title, mi.feed_position, mi.date_published
+FROM collection_items ci
+JOIN media_items mi ON ci.media_item_id = mi.id
+WHERE ci.collection_id = $1
+  AND mi.source = 'podcast_episode'
+  AND mi.download_state = 'downloaded'
+  AND mi.archived_at IS NULL
+  AND mi.missing_scan_count = 0
+ORDER BY ci.position ASC NULLS LAST, ci.added_at ASC;
+
 -- Get collections for book
 -- name: GetCollectionsForBook :many
 SELECT c.*