Make all migration SQL files idempotent for clean prod deploys

Added IF NOT EXISTS, IF EXISTS, and DO $$ EXCEPTION guards to all
migration files from 20260205 onwards so they survive partial application
and work correctly on both fresh databases and existing deployments.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

prisma/migrations/20260211120000_add_award_vote_composite_index/migration.sql

@@ -1,2 +1,2 @@
 -- CreateIndex
-CREATE INDEX "AwardVote_awardId_userId_idx" ON "AwardVote"("awardId", "userId");
+CREATE INDEX IF NOT EXISTS "AwardVote_awardId_userId_idx" ON "AwardVote"("awardId", "userId");