feat(podcasts): generic-surface exclusions — podcasts never render as books (task 1.3)

§9 / D3 / D13 / D15 / D16: podcasts-type libraries are excluded from
every generic book surface. Under RLS these are pure UX/type filters
(engine already hides other users' episodes); their job is preventing
episode grids in surfaces designed for books. The discriminator is the
denormalized media_items.library_type_name (IS DISTINCT FROM 'podcasts'
— NULL rows predate the column and stay visible).

SQL (queries.sql, marked '-- podcasts exclusion (plan §9)'):
ListMediaItems/ListMediaItemsSorted (All Books) · SearchMediaItems +
SearchMediaItemsUnified (generic search) · FacetCountsUnderFilters (all
14 arms) + the six Search*Values facet-suggestion queries ·
GetDistinctSeries + Count (series pages) · GetLibraryItems (collections
picker) · dashboard system sections (Continue Reading / Recently Added
/ Recently Read / Not Started) · GetPopularBooks + GetTopBooksByMinutes
(book rankings must not rank episodes).

Go: statistics summary counts episodes' sessions into time-spent (D15
keeps that) but skips them in BooksActive/BooksFinished — an episode is
not a book. OPDS filters podcasts libraries via one helper at all 7
library-walk sites (D16).

Templ: LibrarySwitcher skips podcasts entries and TotalMediaCount
excludes them — presentation-layer filter so every switcher consumer
(dashboard, collections, custom-section builder) is correct at once
(D13); podcasts surface via /podcasts only.

internal/database/querier.go

@@ -606,6 +606,9 @@ type Querier interface {
 	// collapsible "Ignored" section so a misclick is recoverable.
 	ListIgnoredUnlinkedBooksForUser(ctx context.Context, userID pgtype.UUID) ([]ListIgnoredUnlinkedBooksForUserRow, error)
 	ListLibraries(ctx context.Context) ([]ListLibrariesRow, error)
+	// Podcasts exclusion (plan §9): generic book surfaces never list
+	// podcast episodes (library_type_name is the denormalized, trigger/
+	// fetcher-maintained discriminator; NULL rows predate it and stay).
 	ListMediaItems(ctx context.Context, arg ListMediaItemsParams) ([]ListMediaItemsRow, error)
 	ListMediaItemsByLibrary(ctx context.Context, libraryID pgtype.UUID) ([]ListMediaItemsByLibraryRow, error)
 	ListMediaItemsByLibraryIncludingArchived(ctx context.Context, libraryID pgtype.UUID) ([]ListMediaItemsByLibraryIncludingArchivedRow, error)