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;

prisma/schema.prisma

@@ -86,9 +86,9 @@ model Issue {
   status     String
   priority   Int       @default(0)
   assigneeId String?
-  assignee   User?     @relation(name: "assigneeIssues", fields: [assigneeId], references: [id], onDelete: Cascade)
-  reporterId String
-  reporter   User      @relation(name: "reporterIssues", fields: [reporterId], references: [id], onDelete: Cascade)
+  assignee   User?     @relation(name: "assigneeIssues", fields: [assigneeId], references: [id], onDelete: SetNull)
+  reporterId String?
+  reporter   User?     @relation(name: "reporterIssues", fields: [reporterId], references: [id], onDelete: SetNull)
   projectId  String
   project    Project   @relation(fields: [projectId], references: [id], onDelete: Cascade)
   dueDate    DateTime? @db.Timestamptz(3)