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/router/frontend.go

@@ -752,13 +752,8 @@ func registerFrontendRoutes(cfg *Config) {
 				if collErr != nil {
 					books = handlers.BookCardsFromCollectionDashRows(nil, libTypes)
 				} else {
-					var validItems []database.GetCollectionItemsForDashboardRow
-					for _, item := range collItems {
-						if !item.Excluded.Valid || !item.Excluded.Bool {
-							validItems = append(validItems, item)
-						}
-					}
-					books = handlers.BookCardsFromCollectionDashRows(validItems, libTypes)
+					// The query filters excluded rows in SQL.
+					books = handlers.BookCardsFromCollectionDashRows(collItems, libTypes)
 				}
 			} else {
 				collItems, collErr := cfg.Queries.GetCollectionItems(c.Request().Context(), pgtype.UUID{Bytes: collUUID, Valid: true})