fix(db): add missing Review.userId index for FK query performance

The Review table was missing an index on userId, causing full table scans
when querying reviews by user. All other FK columns across 22 models were
verified to have proper indexes already (including Listing.sellerId which
was added in a prior migration).

Co-Authored-By: Paperclip <noreply@paperclip.ing>

prisma/schema.prisma

@@ -567,4 +567,5 @@ model Review {
   createdAt  DateTime @default(now())
 
   @@index([targetType, targetId])
+  @@index([userId])
 }