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/application/dto/issue-with-users.dto.ts

@@ -8,9 +8,9 @@ export class IssueWithUsersDto {
   status: string;
   priority: number;
   assigneeId: string | null;
-  reporterId: string;
+  reporterId: string | null;
   assignee: UserDto | null;
-  reporter: UserDto;
+  reporter: UserDto | null;
   projectId: string;
   dueDate: Date | null;
   createdAt: Date;

src/modules/issue/application/dto/issue.dto.ts

@@ -6,7 +6,7 @@ export class IssueDto {
   status: string;
   priority: number;
   assigneeId: string | null;
-  reporterId: string;
+  reporterId: string | null;
   projectId: string;
   dueDate: Date | null;
   createdAt: Date;

src/modules/issue/application/mappers/issue-application.mapper.ts

@@ -23,11 +23,6 @@ export class IssueApplicationMapper {
 
   /** Раскрывает assignee/reporter в полноценных пользователей (см. GetIssueUseCase) */
   static toDtoWithUsers(issue: Issue, usersById: ReadonlyMap<string, UserDto>): IssueWithUsersDto {
-    const reporter = usersById.get(issue.reporter);
-    if (!reporter) {
-      throw new Error(`Reporter user ${issue.reporter} not found for issue ${issue.key}`);
-    }
-
     return {
       id: issue.id as number,
       key: issue.key,
@@ -38,7 +33,7 @@ export class IssueApplicationMapper {
       assigneeId: issue.assignee,
       reporterId: issue.reporter,
       assignee: issue.assignee ? (usersById.get(issue.assignee) ?? null) : null,
-      reporter,
+      reporter: issue.reporter ? (usersById.get(issue.reporter) ?? null) : null,
       projectId: issue.projectId,
       dueDate: issue.dueDate,
       createdAt: issue.createdAt,