Add MJPEG camera server with systemd deployment

- camera/capture.py: V4L2 mmap-based MJPEG frame capture with background thread
- camera/mjpeg.py: MJPEG streaming server (multipart/x-mixed-replace) + minimal web UI
- camera/app.py: Entry point with argument parsing and signal handling
- pyproject.toml: uv project config (no external dependencies)
- camera-webui.service: systemd unit for native Pi deployment
- README.md: Updated for systemd-only direction with install instructions

camera-webui.service

@@ -0,0 +1,16 @@
+[Unit]
+Description=USB Camera Web UI
+After=network-online.target
+Wants=network-online.target
+
+[Service]
+Type=exec
+User=mikkeli
+WorkingDirectory=/home/mikkeli/dev/usb-camera-webui
+ExecStartPre=/usr/bin/python3 -c "import v4l2"
+ExecStart=/usr/bin/python3 -m camera.app --host 0.0.0.0 --port 8080
+Restart=on-failure
+RestartSec=5
+
+[Install]
+WantedBy=multi-user.target