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>

src/modules/issue/domain/entities/issue.entity.ts

@@ -12,7 +12,8 @@ interface IssueProps {
   status: IssueStatus;
   priority: IssuePriority;
   assignee: string | null;
-  reporter: string;
+  /** null — автор задачи удалил аккаунт */
+  reporter: string | null;
   projectId: string;
   dueDate: Date | null;
   createdAt: Date;
@@ -72,7 +73,7 @@ export class Issue {
     status: string;
     priority: number;
     assignee: string | null;
-    reporter: string;
+    reporter: string | null;
     projectId: string;
     dueDate: Date | null;
     createdAt: Date;
@@ -122,7 +123,7 @@ export class Issue {
     return this.props.assignee;
   }
 
-  get reporter(): string {
+  get reporter(): string | null {
     return this.props.reporter;
   }