refactor(cards): smart-collection preview through the builder

POST preview-feed had the ladder's 14th hand-rolled literal. Now
BookCardsFromMediaItems + one LibraryTypeMap per request; the response
carries format_group/badge values for the first time (all seven keys
were already emitted as zero-value JSON, so this is a value change on
a pin-frozen shape, not a key change).

internal/handlers/collections.go

@@ -1251,16 +1251,7 @@ func (h *CollectionHandler) PreviewCollection(c *echo.Context) error {
 		matchedItems = matchedItems[:req.Limit]
 	}
 
-	bookCards := make([]BookInfo, len(matchedItems))
+	bookCards := BookCardsFromMediaItems(matchedItems, LibraryTypeMap(c.Request().Context(), h.db))
-	for i, item := range matchedItems {
-		itemUUID, _ := uuid.FromBytes(item.ID.Bytes[0:16])
-		bookCards[i] = BookInfo{
-			MediaItemID:    itemUUID.String(),
-			Title:          item.Title,
-			Author:         textToString(item.Author),
-			CoverImagePath: utils.ResolveMediaURL(item.LibraryID, item.CoverImagePath),
-		}
-	}
 
 	return c.JSON(http.StatusOK, map[string]interface{}{"items": bookCards})
 }