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-regen/route.ts

@@ -272,7 +272,7 @@ export const POST = withAdmin(
 				// Verify the furnidata matches the DB ids
 				const furniReconcile = await reconcileFurniDataWithItemsBase();
 
-				// Rebuild every catalog offer_id from the furnidata
+				// Force every catalog offer_id to equal its own row id
 				const offerRebuild = await rebuildCatalogOfferIds();
 
 				// Repair any sprite_id drift so the client renders the right item