Install missing reader libraries on first start

A freshly flashed device needed manual pip work before the reader would
start. It now checks its dependencies, installs whatever is missing, and
carries on.

- bootstrap.py: import check, pip invocation, and a re-check that does not
  trust pip's exit code alone. Installs nothing when the libraries are
  already present, so a normal start does no network work.
- --wheel-dir installs from local wheels with --no-index, for devices with
  no internet. --no-install-deps opts out entirely.
- Outside a venv pip gets --break-system-packages, which Debian and
  Raspberry Pi OS require before touching the system Python.
- reload_library() re-imports rdm6300 when it was installed at runtime, since
  the module-level import already ran and failed.
- A failed install reports exit 2 and the exact manual command rather than
  raising; stdout is flushed so the explanation is not printed after the
  error when piped.

deploy/rdm-card-reader.service

@@ -18,8 +18,10 @@ Group=dialout
 WorkingDirectory=/opt/rdm-card-reader
 
 # --serve exposes the card on http://127.0.0.1:8765/api/card
+# --port points at the udev alias, so the reader is the same device every boot.
 # Autodetect refuses to guess if more than one USB adapter is present, which is
 # the safe behaviour: a wrong guess means authenticating the wrong person.
+# Add --wheel-dir /opt/rdm-card-reader/wheels to install dependencies offline.
 ExecStart=/opt/rdm-card-reader/.venv/bin/rdm-card-reader \
     --serve \
     --port /dev/rdm-reader \