feat: add YuNet detector option, multi-scale detection, and streamlined CLI

- Add YuNet face detector as alternative option (built into OpenCV)
- Add multi-scale detection (1.0x + 1.5x) to catch faces at different distances
- Add NMS to remove duplicate detections from multi-scale
- Move frame interval and clustering settings to advanced options
- Increase default blur padding from 25% to 40%
- Change default frame interval from 30 to 15
- Change default confidence threshold from 0.7 to 0.8
- Add limitations section to README (extreme angles, small faces, motion blur)
- Require scikit-learn>=1.3.0 for HDBSCAN support

README.md

@@ -51,3 +51,9 @@ The original proof-of-concept command-line interface is also still available for
 ```bash
 uv run pyfaceblur-legacy detect --video input.mp4 --output ./output --interval 30 --confidence 0.7
 ```
+
+## Limitations
+
+- **Extreme face angles:** Faces viewed from extreme angles (e.g., strong profile views, looking up/down) may not be detected or may be clustered as separate identities. For best results, use videos where faces are mostly front-facing or at moderate angles.
+- **Small/distant faces:** Very small faces (below 50 pixels) may not be reliably detected or produce accurate embeddings for clustering.
+- **Rapid motion blur:** Fast head movements causing motion blur can affect detection accuracy.