feat(phase1): implement OpenCV image processing pipeline

- Create ImageProcessor service with EXIF orientation detection
- Implement smart cropping via OpenCV contour detection (10% padding)
- Add text orientation detection using Hough line transform
- Resize and compress images to 1200px with 85% JPEG quality
- Generate 200px square thumbnails with center crop
- Fallback to Pillow if OpenCV fails
- Comprehensive test suite: 28 tests all passing
- File size validation (reject >10MB)
- Graceful error handling for corrupted/invalid images
- Update requirements.txt with opencv-python, piexif, python-magic

backend/requirements.txt

@@ -17,3 +17,6 @@ pytest>=8.0.0
 pytest-asyncio>=0.23.0
 pytest-cov>=4.1.0
 httpx>=0.27.0
+opencv-python>=4.8.0
+piexif>=1.1.3
+python-magic>=0.4.27