refactor(db): GetCollectionItemsForDashboard embeds the media row; excluded filter moves into SQL; third converter dies

SELECT mi.*, ci.excluded forced every caller to filter tombstone
exclusions in Go over a 69-field row, then a 45-field hand converter
(getCollectionItemsRowToMediaItems) copied it field-by-field into
MediaItems for BuildSections. The query now filters
(ci.excluded IS NOT TRUE) itself and returns sqlc.embed(mi) — callers
consume .MediaItems directly, the Go filter loops die in both
collection-detail ladders (JSON + SSR), and the converter is deleted.

Pins, H1/H3 tests, unit suites green.

internal/database/querier.go

@@ -174,6 +174,9 @@ type Querier interface {
 	GetCollection(ctx context.Context, id pgtype.UUID) (Collections, error)
 	// Get collection items
 	GetCollectionItems(ctx context.Context, collectionID pgtype.UUID) ([]GetCollectionItemsRow, 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).
 	GetCollectionItemsForDashboard(ctx context.Context, arg GetCollectionItemsForDashboardParams) ([]GetCollectionItemsForDashboardRow, error)
 	// Get collection with book count
 	GetCollectionWithBookCount(ctx context.Context, id pgtype.UUID) (GetCollectionWithBookCountRow, error)