[BE-28] Implement document watermarking on download

[mftee]

Overview

Overlay a visible watermark (user name + timestamp) on documents when downloaded inside backend/cmmty/.

Acceptance Criteria

• Create backend/cmmty/watermark/ with module, controller, and service
• Implement GET /cmmty/documents/:id/download returning the watermarked file
• Watermark text: requester name + timestamp + document ID
• Support PDF and image (PNG/JPEG) watermarking
• Write unit tests for watermark text generation

Contribution Folder

All files must be placed inside backend/cmmty/. Do not modify any file outside this folder.

How to Contribute

Comment on this issue to express interest, fork the repo, build inside backend/cmmty/ only, and open a PR targeting main.

[Cedarich]

@Cedarich has applied to work on this issue as part of the Stellar Wave Program's 4th wave.

Hello maintainer, please can i take up this task

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

[drips-wave]

Congratulations, @Cedarich! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on April 29, 2026.

🧑‍💻 @Cedarich: 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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[drips-wave]

This issue has been completed by @Cedarich as part of the Stellar Wave Program's 4th Wave 🥳

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