Add user-drawn bounding box annotations, collections, and review workflow

[garland3]

Summary

• Adds user-drawn bounding box annotation capability with per-project class definitions, interactive drawing tool, and COCO/YOLO export (closes Add user-drawn bounding box annotation capability with per-project classes #22)
• Adds image collections, collection locking, annotation review workflow, and audit trail (closes Add user-drawn bounding boxes, annotation review workflow, and image collections #56)
• 6 new database models, 4 new API routers (26 endpoints), Alembic migration
• 8 new frontend components, 2 custom hooks for state management
• 30 new backend tests (195 total passing)
• Annotation data included in all export formats (CSV, JSON, Excel) and visual report with bbox overlays
• Three interaction modes (Pan/Select/Draw) with keyboard shortcuts and hover-to-highlight

Screenshots

Image Viewer with Bounding Box Annotations

Report with Bbox Overlays on Thumbnails

Report Statistics with Annotation Count

Test plan

• Run alembic upgrade head to apply migration
• Verify bbox class CRUD on project page (create with color, edit, delete)
• Verify drawing bounding boxes on images in the viewer (click-and-drag)
• Verify annotation list in sidebar (select, edit class, delete)
• Verify COCO and YOLO export endpoints return valid formats
• Verify collection CRUD (create, add images, lock/unlock)
• Verify annotation review (approve/reject/flag) and review history
• Verify report page shows bbox overlays on annotated image thumbnails
• Verify CSV/JSON/Excel exports include annotation data columns
• Run full backend test suite: uv run pytest tests/
• Run frontend build: cd frontend && npm run build