Implemented the "PDF Stamp Operation" feature, allowing users to add text, image, or predefined stamps to PDF documents.
- Added `FEATURE_7_PLAN.md` with detailed implementation plan.
- Introduced enums (`StampType`, `PredefinedStampType`, `StampPlacement`) in the domain layer.
- Added `AddStampAsync` method to `IPdfProcessor` interface.
- Implemented `AddStampAsync` in `DevExpressPdfProcessor` using DevExpress API.
- Created unit tests for `AddStampAsync` covering various scenarios.
- Added `AddStampCommand` in the application layer with validation rules.
- Created two new endpoints in `PdfOperationsController` for multipart and Base64 inputs.
- Added DTOs for handling endpoint requests.
- Wrote integration tests for endpoints to ensure correctness.
- Updated `AGENTS.md` and added DevExpress API references.
- Documented challenges, considerations, and estimated effort.
This commit completes the "PDF Stamp Operation" feature with full test coverage and documentation.
