feat: add composite unique index to Hacker table

Ensure unique constraint on userId and hackathonId columns. This prevents duplicate entries and enforces data integrity for hacker-hackathon pairs.

prisma/migrations/20241015134428_change_unique_hacker_to_composite/migration.sql

@@ -0,0 +1,8 @@
+/*
+  Warnings:
+
+  - A unique constraint covering the columns `[userId,hackathonId]` on the table `Hacker` will be added. If there are existing duplicate values, this will fail.
+
+*/
+-- CreateIndex
+CREATE UNIQUE INDEX "Hacker_userId_hackathonId_key" ON "Hacker"("userId", "hackathonId");