IMG-031: No Format Conversion

## Summary
Unsupported formats (HEIC, TIFF) rejected instead of converted.

## Fix
Implement format conversion using sharp library (HEIC→JPEG, TIFF→PNG).

## Acceptance Criteria
- [ ] Implement format conversion using sharp
- [ ] Support HEIC→JPEG conversion
- [ ] Support TIFF→PNG conversion
- [ ] Configurable output format
- [ ] Add tests for format conversion

**Estimated Effort:** 8h