Implement collection item reordering with bulk operations and persistent HTML attributes

- Add bulk reorder API endpoint (PUT /api/collections/{id}/reorder) with atomic transactions
- Replace individual position updates with efficient bulk operations in frontend
- Implement unified ID generation and proper data-item-id injection during enhancement
- Fix collection item position persistence through content edit cycles
- Add optimistic UI with rollback capability for better user experience
- Update sqlc queries to include last_edited_by fields in position updates
- Remove obsolete data-content-type attributes and unify naming conventions

internal/api/models.go

@@ -123,3 +123,13 @@ type UpdateCollectionItemRequest struct {
 	Position    int    `json:"position"`
 	UpdatedBy   string `json:"updated_by,omitempty"`
 }
+
+type CollectionItemPosition struct {
+	ItemID   string `json:"itemId"`
+	Position int    `json:"position"`
+}
+
+type ReorderCollectionRequest struct {
+	Items     []CollectionItemPosition `json:"items"`
+	UpdatedBy string                   `json:"updated_by,omitempty"`
+}