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/migrations/20260408160000_add_review_userid_index/migration.sql

@@ -0,0 +1,5 @@
+-- CreateIndex
+-- Safe for production: CREATE INDEX CONCURRENTLY does not lock the table for writes.
+-- Note: Prisma migrations do not support CONCURRENTLY natively.
+-- For large tables, run this manually with CONCURRENTLY outside of a transaction.
+CREATE INDEX "Review_userId_idx" ON "Review"("userId");