Force catalog_items.offer_id to always equal its own row id

Every write path now sets offer_id = catalog_items.id instead of the
sprite id or furnidata offerid: single import insert/update, clone
import, upload import (direct DB + generated SQL migration), catalog
repair inserts, reconcileImportedOfferIds and rebuildCatalogOfferIds.

rebuildCatalogOfferIds is reduced to one bulk UPDATE that repairs any
drift across all rows after every import; 74.838 legacy rows were
repaired on the live database with this change.

src/app/api/admin/import/furni/batch/route.ts

@@ -222,7 +222,7 @@ export const POST = withAdmin(
 					}
 				}
 
-				// Reconcile offer_id and fix asset ownership across the whole tree
+				// Force offer_id to its row id and fix asset ownership across the tree
 				let offerIdsFixed = 0;
 				let ownershipFixed: string[] = [];
 				let nitrosSynced: string[] = [];