Keep issues when assignee or reporter is deleted

Switch issue.assigneeId/reporterId FKs from CASCADE to SET NULL and make
reporter nullable through the entity, DTOs and GraphQL types.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

prisma/migrations/20261001081818_issue_users_set_null/migration.sql

@@ -0,0 +1,14 @@
+-- DropForeignKey
+ALTER TABLE "issue" DROP CONSTRAINT "issue_assigneeId_fkey";
+
+-- DropForeignKey
+ALTER TABLE "issue" DROP CONSTRAINT "issue_reporterId_fkey";
+
+-- AlterTable
+ALTER TABLE "issue" ALTER COLUMN "reporterId" DROP NOT NULL;
+
+-- AddForeignKey
+ALTER TABLE "issue" ADD CONSTRAINT "issue_assigneeId_fkey" FOREIGN KEY ("assigneeId") REFERENCES "user"("id") ON DELETE SET NULL ON UPDATE CASCADE;
+
+-- AddForeignKey
+ALTER TABLE "issue" ADD CONSTRAINT "issue_reporterId_fkey" FOREIGN KEY ("reporterId") REFERENCES "user"("id") ON DELETE SET NULL ON UPDATE CASCADE;