Add support for external network cameras (MJPEG, RTSP, HTTP snapshot)
that replace a printer's built-in camera when configured.
Features:
- Live streaming on printers page (replaces built-in camera)
- Finish photo capture from external camera on print complete
- Layer-based timelapse: captures frame on each layer change,
stitches to MP4 video on print completion
Backend changes:
- Add external_camera_url, external_camera_type, external_camera_enabled
fields to Printer model with database migration
- New external_camera.py service: MJPEG/RTSP/snapshot frame capture,
connection testing, MJPEG stream generation
- New layer_timelapse.py service: TimelapseSession management,
layer-by-layer frame capture, ffmpeg video stitching
- Add on_layer_change callback to MQTT client and printer manager
- Update camera routes with external camera streaming and tracking
- Update print lifecycle hooks for timelapse start/stitch/cancel
- Add external camera fields to backup/restore
- Rate limiting for external camera streams (prevents browser freeze)
Frontend changes:
- Add external camera configuration UI in Settings > Camera
- Per-printer enable toggle, URL input, type selector, test button
- Toast notification on save
Closes#143
