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/handlers/cards.go

@@ -43,6 +43,18 @@ type BookInfo struct {
 	// it empty rather than pay a join).
 	ViewerPercentage *float64 `json:"viewer_percentage"`
 	LibraryName      string   `json:"library_name"`
+	// Episode-collections extras (plan item 2.3): populated only by the
+	// collection-detail adapter. IsEpisode renders the episode-row
+	// variant in the grid; FeedTitle disambiguates same-named episodes;
+	// DownloadState gates the play affordance. Hidden=true means the
+	// LEFT JOIN missed under RLS (unsubscribed episode, or a
+	// family-lens hidden book) — the row renders as an "unavailable"
+	// hint instead of silently vanishing. Adapters that don't load
+	// visibility leave Hidden false (cards stay fully rendered).
+	IsEpisode     bool   `json:"is_episode,omitempty"`
+	FeedTitle     string `json:"feed_title,omitempty"`
+	DownloadState string `json:"download_state,omitempty"`
+	Hidden        bool   `json:"hidden,omitempty"`
 }
 
 // LibraryTypeMap builds libraryID→library-type-name for badge resolution
@@ -151,8 +163,7 @@ func BookCardsFromCollectionDashRows(rows []database.GetCollectionItemsForDashbo
 			ID:          uuid.UUID(mi.ID.Bytes),
 			Title:       mi.Title,
 			Author:      utils.PgTextString(mi.Author),
-			LibraryID:   mi.LibraryID,
-			CoverPath:   mi.CoverImagePath,
+			LibraryID:   mi.LibraryID,			CoverPath:   mi.CoverImagePath,
 			FormatGroup: mi.FormatGroup,
 			LibraryType: libTypes[uuid.UUID(mi.LibraryID.Bytes).String()],
 		}))
@@ -180,6 +191,39 @@ func BookCardsFromCollectionItemRows(rows []database.GetCollectionItemsRow, libT
 	return cards
 }
 
+// BookCardsFromCollectionDetailed builds cards from the
+// GetCollectionItemsDetailed rows (plan item 2.3): the collection's
+// full membership with per-row visibility. RLS-invisible rows
+// (unsubscribed episode, family-lens hidden book) become Hidden cards
+// — the grid renders them as "unavailable" hint rows; episode rows
+// carry feed title + download state for the play affordance. Unknown
+// library types resolve no badge; hidden rows need none anyway.
+func BookCardsFromCollectionDetailed(rows []database.GetCollectionItemsDetailedRow, libTypes map[string]string) []BookInfo {
+	cards := make([]BookInfo, 0, len(rows))
+	for _, r := range rows {
+		if !r.Visible {
+			cards = append(cards, BookInfo{
+				MediaItemID: uuid.UUID(r.MediaItemID.Bytes).String(),
+				Hidden:      true,
+			})
+			continue
+		}
+		isEpisode := r.Source.String == "podcast_episode"
+		card := buildBookCard(cardSource{
+			ID:          uuid.UUID(r.MediaItemID.Bytes),
+			Title:       utils.PgTextString(r.Title),
+			Author:      utils.PgTextString(r.Author),
+			CoverPath:   r.CoverImagePath,
+			FormatGroup: r.FormatGroup.String,
+		})
+		card.IsEpisode = isEpisode
+		card.FeedTitle = r.FeedTitle.String
+		card.DownloadState = r.DownloadState.String
+		cards = append(cards, card)
+	}
+	return cards
+}
+
 // BookCardsFromUnified builds cards from search-unified rows. Those
 // rows carry the library type name as a SQL column, so this adapter
 // needs no LibraryTypeMap query at all — and the viewer's own