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/handlers/collections.go

@@ -293,13 +293,9 @@ func (h *CollectionHandler) GetCollection(c *echo.Context) error {
 		if collErr != nil {
 			bookList = BookCardsFromCollectionDashRows(nil, libTypes)
 		} else {
-			var validItems []database.GetCollectionItemsForDashboardRow
-			for _, item := range collItems {
-				if !item.Excluded.Valid || !item.Excluded.Bool {
-					validItems = append(validItems, item)
-				}
-			}
-			bookList = BookCardsFromCollectionDashRows(validItems, libTypes)
+			// The query filters excluded rows in SQL; cards build straight
+			// from the embedded media rows.
+			bookList = BookCardsFromCollectionDashRows(collItems, libTypes)
 		}
 	} else {
 		books, booksErr := h.GetCollectionBooksData(c, collectionID)