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/app.py

@@ -0,0 +1,36 @@
+"""Entry point for the USB camera web UI server.
+
+Usage::
+
+    uv run python -m camera.app            # run directly
+    uv run python -m camera.app --host 0.0.0.0 --port 8080
+"""
+
+import argparse
+import signal
+import sys
+
+from camera.mjpeg import run
+
+
+def main():
+    parser = argparse.ArgumentParser(description="USB Camera Web UI Server")
+    parser.add_argument("--host", default="0.0.0.0", help="Bind address (default: 0.0.0.0)")
+    parser.add_argument("--port", type=int, default=8080, help="Port (default: 8080)")
+    args = parser.parse_args()
+
+    # Handle shutdown gracefully
+    def _signal_handler(signum, frame):
+        print(f"\nReceived signal {signum}, shutting down...")
+        sys.exit(0)
+
+    signal.signal(signal.SIGINT, _signal_handler)
+    signal.signal(signal.SIGTERM, _signal_handler)
+
+    print(f"Starting camera web UI on http://{args.host}:{args.port}")
+    print("Press Ctrl+C to stop")
+    run(host=args.host, port=args.port)
+
+
+if __name__ == "__main__":
+    main()