New page /event/{id}/annotate:
- Frame selector grid (all frames from the event)
- Canvas with click-and-drag rectangle drawing to mark the plate bbox
- Touch support for tablets
- Plate text input (pre-filled with auto-detected plate)
- Saves image + YOLO label (.txt) + plates.csv to /data/annotations/
- Stores annotation metadata in DB (annotations table)
/dataset/export serves a ZIP with images/, labels/, plates.csv and
a data.yaml ready for YOLOv8/v9 fine-tuning on Google Colab.
"Annoter" button added to event detail header.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
