Fix V4L2 mmap capture and service configuration

- capture.py: use mmap.mmap instead of non-existent os.mmap
- capture.py: fix v4l2_fourcc() to pass 4 char arguments
- camera-webui.service: add Group=video and PATH env var

camera-webui.service

@@ -6,8 +6,9 @@ Wants=network-online.target
 [Service]
 Type=exec
 User=mikkeli
+Group=video
 WorkingDirectory=/home/mikkeli/dev/usb-camera-webui
-ExecStartPre=/usr/bin/python3 -c "import v4l2"
+Environment=PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
 ExecStart=/usr/bin/python3 -m camera.app --host 0.0.0.0 --port 8080
 Restart=on-failure
 RestartSec=5

camera/capture.py

@@ -1,16 +1,20 @@
-"""V4L2 MJPEG capture via the v4l2 Python library (mmap).
+"""V4L2 MJPEG capture via mmap buffers.
 
-Uses memory-mapped buffers for efficient frame capture without subprocess
-overhead. Each frame is a complete JPEG image ready for MJPEG streaming.
+Opens /dev/video0 and uses memory-mapped V4L2 buffers for efficient frame
+capture. Each frame is a complete JPEG image ready for MJPEG streaming.
 """
 
 import fcntl
 import os
 import time
 import threading
+import mmap
 
 import v4l2
 
+# Fourcc for Motion JPEG
+MJPG = v4l2.v4l2_fourcc("M", "J", "P", "G")
+
 # Global camera singleton
 _camera: "Capture | None" = None
 _camera_lock = threading.Lock()
@@ -79,10 +83,10 @@ class Capture:
         fmt.type = v4l2.V4L2_BUF_TYPE_VIDEO_CAPTURE
         fmt.fmt.pix.width = self._width
         fmt.fmt.pix.height = self._height
-        fmt.fmt.pix.pixelformat = v4l2.v4l2_fourcc("MJPG")
+        fmt.fmt.pix.pixelformat = MJPG
         fmt.fmt.pix.field = v4l2.V4L2_FIELD_ANY
         _ioctl(self._fd, v4l2.VIDIOC_S_FMT, fmt)
-        print(f"Format set: {fmt.fmt.pix.width}x{fmt.fmt.pix.height} {v4l2.v4l2_fourcc2str(fmt.fmt.pix.pixelformat)}")
+        print(f"Format: {fmt.fmt.pix.width}x{fmt.fmt.pix.height} {v4l2.v4l2_fourcc2str(fmt.fmt.pix.pixelformat)}")
 
     def _allocate_buffers(self) -> None:
         req = v4l2.v4l2_requestbuffers()
@@ -101,7 +105,7 @@ class Capture:
 
             length = buf.length
             offset = buf.m.offset
-            data = os.mmap(0, length, os.PROT_READ, os.MAP_SHARED, self._fd, offset)
+            data = mmap.mmap(self._fd, length, mmap.MAP_SHARED, mmap.PROT_READ, 0, offset)
             self._buffers.append({
                 "index": i,
                 "length": length,