[Enhancement]: Add GET /api/documents/:id endpoint for single document retrieval #62

🧭 Project Area

Backend / API

📝 Issue Summary and Problem Description

Issue summary:
The backend document module provides endpoints for listing (GET /api/documents), uploading (POST /api/documents), verifying (PATCH /api/documents/:id/verify), and deleting (DELETE /api/documents/:id). However, there is no endpoint to fetch details of a single document by its ID.

Problem description:
If a client attempts to fetch metadata for a single document via GET /api/documents/:id, the request fails with a generic HTTP 404 ROUTE_NOT_FOUND error instead of a resource lookup response.

Why this is important:
Single resource retrieval is a core RESTful API convention required for document detail pages, auditing, and deep linking within the CertiVault platform.

Who is affected:
Frontend developers building document preview modals or detail views, and API consumers needing single document metadata.

🔍 Current and Expected Behaviour

Current behaviour:
GET /api/documents/demo-credential returns HTTP 404 ROUTE_NOT_FOUND ("Route GET /api/documents/demo-credential was not found").

Current result:
Single document details cannot be queried individually by ID.

Expected behaviour:
GET /api/documents/:id should look up the document in documentStore. If found, return HTTP 200 with { data: document }. If not found, return HTTP 404 with DOCUMENT_NOT_FOUND.

Expected result:
GET /api/documents/demo-credential returns 200 OK with the document payload.

🧩 Reproduction and Evidence

Step 1: Start backend (`npm run dev`)
Step 2: Send GET request to `http://localhost:5000/api/documents/demo-credential`
Step 3: Observe 404 ROUTE_NOT_FOUND error
Step 4: Check document.routes.js — no GET /:id route exists

🎯 Affected Scope

Affected files:

• backend/src/modules/documents/document.controller.js
• backend/src/modules/documents/document.routes.js
• backend/tests/integration/app.test.js

Affected APIs: GET /api/documents/:id
Database changes required? No

🖥 Environment

Operating system: Windows 11
Node.js version: v22.x
Package manager: npm 10.x
Deployment environment: Local development

💡 Solution, Alternatives, and Estimate

Priority: P2 Medium
Proposed solution:

1. Export getDocument controller function in document.controller.js using documentStore.find(req.params.id).
2. Register documentRouter.get("/:id", getDocument) in document.routes.js.
3. Add integration tests verifying HTTP 200 (success path) and HTTP 404 (non-existent ID).

Alternative solutions: Client could filter the full GET /api/documents array, but this is inefficient and bad for bandwidth as document count grows.
Difficulty: Beginner
Estimated time: 30 minutes
Dependencies or blockers: None

✅ Acceptance Criteria and Additional Notes

• GET /api/documents/:id returns 200 OK with document data when ID exists
• GET /api/documents/:id returns 404 DOCUMENT_NOT_FOUND when ID does not exist
• Integration tests added to app.test.js covering both success and 404 cases
• All existing 18 tests continue to pass

🤝 Contributor Checklist

• I have searched existing issues and pull requests for duplicates.
• I can reproduce the issue, or I have clearly explained why reproduction is not applicable.
• I will follow the contribution guidelines and target ecsoc-2026.
• I agree to follow the Code of Conduct.
• I have removed secrets, private documents, and personal data.