[BE-14] Create DepartmentsModule with CRUD endpoints in opsce module

[yusuftomilola]

Problem

Departments cannot be managed via the API. Assets cannot be properly assigned to departments, breaking the organizational hierarchy feature.

Proposed Solution

Create backend/src/opsce/departments/departments.controller.ts and backend/src/opsce/departments/departments.service.ts.

Acceptance Criteria

• POST /api/departments creates a department, optionally with a parentId (ADMIN only)
• GET /api/departments returns all departments as a flat list with a childCount field
• GET /api/departments/:id returns a department with its children and asset count
• PATCH /api/departments/:id updates name, description, or parent (ADMIN only)
• DELETE /api/departments/:id fails with 409 if the department has active assets
• CreateDepartmentDto validates name as required string, parentId as optional UUID

[Jacques-codes]

@Jacques-codes has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Hiii, Good Morning Ser, I love your product, and I would love to solve this issue and solve it accurately.

Thank you for granting my request.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Jacques-codes to this issue.

[Alimzy]

@Alimzy has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Hi, I'd like to work on this.
I'll create departments.service.ts and departments.controller.ts inside backend/src/opsce/departments/, with a Department TypeORM entity supporting a self-referential parentId for hierarchy. The service will handle POST /api/departments (ADMIN-guarded, optional parentId) and GET /api/departments (flat list with a childCount derived via a subquery). The module gets registered in the OpsceModule.
Can I be assigned?

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Alimzy to this issue.

[KuchiMercy]

@KuchiMercy has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Hi Maintainer, I'd like to work on this issue. Can you assign?

ℹ️ Repo Maintainers: To accept this application, review their application or assign @KuchiMercy to this issue.

[Jerry-Tekh]

@Jerry-Tekh has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Hi maintener, I can work and get this done within the required time.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Jerry-Tekh to this issue.

[drips-wave]

Congratulations, @KuchiMercy! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 2, 2026.

🧑‍💻 @KuchiMercy: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

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