Fix install.sh failing on offline Raspberry Pi at the venv step

install.sh ran 'pip install .', which builds the project: pip downloads
setuptools into an isolated build environment. With no internet it fails at
the 'Building the virtual environment' step with

    Could not find a version that satisfies the requirement setuptools>=68
    (from versions: none)

even though the runtime libraries are already in the wheel directory. The
build backend is only needed to package, never to run.

- Install the runtime deps straight from the bundled wheels with --no-index,
  and put src/ on the path via a .pth file, so no build step happens at all.
- Generate the console script with the venv's interpreter as an absolute
  shebang. '#\!/usr/bin/env python3' resolves to the *system* python when the
  venv is not activated, so the package was not importable from the unit.
- apt-get update/install and 'pip install --upgrade pip' are now best-effort:
  all three need the network and none should abort the install.
- Detect a failed 'python3 -m venv' and name python3-venv as the usual cause.
- Sanity-check the install with --version before enabling the service.
- The unit falls back to 'python -m rdm_card_reader' if the console script is
  ever absent (no shell in ExecStart, so the installer picks the path).

Reproduced the failure offline, then verified the fix offline as well: a venv
created on a copy of the tree self-provisions from the bundled wheels and runs
from cwd=/ with no PYTHONPATH and no venv activation.

Added tests/test_deploy.py (20 tests) covering the installer and the deploy
artifacts, including a regression test for 'pip install .' that was confirmed
to fail when the bug is reintroduced.

README.md

@@ -53,18 +53,45 @@ without its library is worse than one that refuses to start.
 
 ## Install
 
+On a machine **with internet**:
+
 ```bash
 pip install .
 ```
 
-On a Raspberry Pi OS install with an externally-managed Python, add
-`--break-system-packages` or use a virtual environment:
+`rdm6300` and `pyserial` are pulled in automatically. On Raspberry Pi OS with
+an externally-managed Python, add `--break-system-packages` or use a venv:
 
 ```bash
 python3 -m venv .venv && . .venv/bin/activate && pip install .
 ```
 
-`rdm6300` and `pyserial` are pulled in automatically.
+### On a device with no internet
+
+`pip install .` will **not** work offline. It builds the project, which makes
+pip download `setuptools` into an isolated build environment, and that fails
+with:
+
+```
+ERROR: Could not find a version that satisfies the requirement setuptools>=68
+       (from versions: none)
+```
+
+The runtime dependencies are bundled in `src/rdm_card_reader/wheels/`, so
+install those directly and put `src` on the path — no build step, no network:
+
+```bash
+python3 -m venv .venv
+.venv/bin/pip install --no-index \
+    --find-links src/rdm_card_reader/wheels rdm6300 pyserial
+echo "$PWD/src" > "$( .venv/bin/python -c \
+    'import sysconfig; print(sysconfig.get_paths()["purelib"])' )/rdm_card_reader_src.pth"
+.venv/bin/python -m rdm_card_reader --demo     # verify, no hardware needed
+```
+
+`deploy/install.sh` does exactly this, plus the service setup. Running the
+app directly also works: it installs anything missing from the same bundled
+wheels on first start.
 
 ---