Add camera selection diagnostics

resolve_device now prints each by-id candidate it scans, which entry
matched, and the resolved device path; the no-match error lists all
by-id entries with their targets. Camera startup logs the raw config
setting, resolved path, the node's sysfs name, the OpenCV version, and
the negotiated resolution. 'python3 camera.py' now dumps every detected
camera before opening, to troubleshoot machines where the wrong camera
gets used.

README.md

@@ -109,6 +109,12 @@ Test the camera on its own (uses the device from `config.toml`):
 python3 camera.py
 ```
 
+It first prints a pre-flight dump of every camera the system sees
+(`/dev/v4l/by-id/` names, device nodes, and what each node identifies as),
+then reports which device was matched, resolved, and opened, plus the OpenCV
+version. If the booth ever uses the wrong camera, run this and read the
+output — it shows exactly where the selection went sideways.
+
 **If it reports black frames** (or the live preview shows a warning): the
 Macally cam's chipset sometimes wedges and streams pure black until it is
 power-cycled. **Unplug the camera, plug it back in, and restart the