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/router/helpers.go

@@ -331,11 +331,17 @@ func (cfg *Config) collectionBooksForScope(c *echo.Context, userUUID uuid.UUID,
 				books = handlers.BookCardsFromCollectionDashRows(collItems, libTypes)
 			}
 		} else {
-			collItems, collErr := cfg.Queries.GetCollectionItems(c.Request().Context(), collUUID)
+			// Episode collections (plan item 2.3): the detailed query
+			// keeps RLS-invisible rows as Hidden cards (unsubscribed
+			// episodes render an "unavailable" hint) and tags episode
+			// rows with feed title + download state. Same position
+			// ordering; the legacy INNER-JOIN shape remains on the
+			// library-scoped branch above.
+			collItems, collErr := cfg.Queries.GetCollectionItemsDetailed(c.Request().Context(), collUUID)
 			if collErr != nil {
 				collItems = nil
 			}
-			books = handlers.BookCardsFromCollectionItemRows(collItems, libTypes)
+			books = handlers.BookCardsFromCollectionDetailed(collItems, libTypes)
 		}
 	}
 	return books, isSmart, ""