Backend: Scan processing and anchor management endpoints

Implement backend API endpoints to support the scan lifecycle, including:
- POST /api/scans/request-upload: Initiate scan record, return upload targets.
- POST /api/scans/:id/upload: Accept/upload scan file (GLB/USDZ, demo endpoint or S3 presign pattern).
- POST /api/scans/:id/complete: Mark scan as ready for AI processing.
- GET /api/scans/:id: Fetch scan metadata, anchor points, and preview URLs.
- POST /api/anchors/:anchorId/link: Attach item to an anchor.
- POST /api/scans/:id/anchors: List/add anchors for scans.

Requirements:
- Local (demo) storage and S3-ready abstraction for uploads.
- Basic in-memory or DB persistence for scans and anchors.
- Validate uploads, return errors if missing or wrong format files.
- Clear error handling and status fields (draft, uploaded, processed, error).

Acceptance:
- All REST API endpoints as described.
- Proper error responses and lifecycle transitions.
- Demo runs in Docker Compose with ai service.