# Change: Add Watermark Feature

## Why
MED-009: The `Dynamic Watermarking` requirement exists in the spec but is not implemented. Downloaded files do not contain user watermarks, making it impossible to trace document leaks.

## What Changes
- Create WatermarkService for image and PDF watermarking
- Integrate watermark generation into download flow
- Support configurable watermark content (name, employee ID, timestamp)

## Impact
- Affected specs: document-management
- Affected code:
  - `backend/app/services/watermark_service.py` (new)
  - `backend/app/api/attachments/router.py` (modify download endpoint)
  - `backend/requirements.txt` (add Pillow, PyMuPDF)