feat: prevent duplicate award pool notifications

Add notifiedAt field to AwardEligibility. notifyEligibleProjects now
skips already-notified entries and stamps notifiedAt after sending,
so re-clicking "Notify Pool" only emails newly added projects.

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

prisma/migrations/20260303232956_add_award_eligibility_notified_at/migration.sql

@@ -0,0 +1,2 @@
+-- AlterTable
+ALTER TABLE "AwardEligibility" ADD COLUMN     "notifiedAt" TIMESTAMP(3);

prisma/schema.prisma

@@ -1622,6 +1622,9 @@ model AwardEligibility {
   confirmedAt DateTime?
   confirmedBy String?
 
+  // Pool notification tracking
+  notifiedAt DateTime?
+
   createdAt DateTime @default(now())
   updatedAt DateTime @updatedAt