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/querier.go

@@ -262,11 +262,25 @@ type Querier interface {
 	// rename can't spawn duplicates on the next scan.
 	GetCollectionByImportedSeries(ctx context.Context, arg GetCollectionByImportedSeriesParams) (Collections, error)
 	GetCollectionByUserAndName(ctx context.Context, arg GetCollectionByUserAndNameParams) (Collections, error)
+	// 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).
+	GetCollectionEpisodesOrdered(ctx context.Context, collectionID pgtype.UUID) ([]GetCollectionEpisodesOrderedRow, error)
 	// Get collection items
 	// A2c: position-first ordering (playlist reading order); un-positioned
 	// legacy items keep added_at ASC after the positioned tail — NOT DESC,
 	// which walked a playlist backwards.
 	GetCollectionItems(ctx context.Context, collectionID pgtype.UUID) ([]GetCollectionItemsRow, error)
+	// 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).
+	GetCollectionItemsDetailed(ctx context.Context, collectionID pgtype.UUID) ([]GetCollectionItemsDetailedRow, error)
 	// The excluded filter lives here (was: callers filtered ci.excluded in
 	// Go over a 69-field row + a 45-field converter to MediaItems; embed
 	// returns the real row so neither exists).
@@ -762,6 +776,14 @@ type Querier interface {
 	RevokeOpdsToken(ctx context.Context, token string) error
 	RevokeRefreshToken(ctx context.Context, token pgtype.UUID) error
 	SearchAuthorValues(ctx context.Context, arg SearchAuthorValuesParams) ([]SearchAuthorValuesRow, error)
+	// 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.
+	SearchEpisodesForPicker(ctx context.Context, q_ string) ([]SearchEpisodesForPickerRow, error)
 	SearchGenreValues(ctx context.Context, arg SearchGenreValuesParams) ([]SearchGenreValuesRow, error)
 	SearchLanguageValues(ctx context.Context, arg SearchLanguageValuesParams) ([]SearchLanguageValuesRow, error)
 	// Search Media Items queries