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.

LAUNCH.md

@@ -30,6 +30,70 @@ All commands run from the project root. The virtualenv is already built.
 ```
 
 After `pip install .` the same thing is available as `rdm-card-reader …`.
+`pip install .` needs the network — see **Troubleshooting** below for the
+offline route.
+
+---
+
+## Troubleshooting install
+
+### "Could not find a version that satisfies the requirement setuptools>=68"
+
+Seen when running `deploy/install.sh` on a Pi with no internet, at the
+"Building the virtual environment" step:
+
+```
+ERROR: Could not find a version that satisfies the requirement setuptools>=68
+       (from versions: none)
+ERROR: No matching distribution found for setuptools>=68
+```
+
+**Cause:** `pip install .` builds the project. pip creates an isolated build
+environment and downloads the *build* backend (`setuptools`) into it — even
+though the *runtime* libraries are already sitting in the wheel directory. With
+no network that download cannot happen.
+
+**It is not** a problem with the bundled wheels, and not a Python version
+mismatch. Two separate confusions produce a similar-looking error:
+
+| Situation | Error |
+| --- | --- |
+| Build isolation, no network | `Could not find a version ... setuptools>=68` |
+| `--no-build-isolation`, and setuptools absent | `Cannot import 'setuptools.build_meta'` |
+
+Note that Python 3.12+ venvs no longer include `setuptools`, so the second case
+is increasingly common.
+
+**Fix:** the current `install.sh` avoids the build entirely — it installs the
+bundled runtime wheels and puts `src/` on the path with a `.pth` file. If you
+are on an older copy, update it (`git pull`) or run these by hand on the Pi:
+
+```bash
+python3 -m venv .venv
+.venv/bin/pip install --no-index \
+    --find-links src/rdm_card_reader/wheels rdm6300 pyserial
+SITE=$(.venv/bin/python -c 'import sysconfig; print(sysconfig.get_paths()["purelib"])')
+echo "$PWD/src" > "$SITE/rdm_card_reader_src.pth"
+.venv/bin/python -m rdm_card_reader --demo     # verify, no hardware needed
+```
+
+No build step, no network, and the same result.
+
+### "Could not create the virtualenv"
+
+On Debian and Raspberry Pi OS, `python3 -m venv` needs the `python3-venv`
+package:
+
+```bash
+sudo apt-get install python3-venv
+```
+
+### apt-get failing on the first step
+
+`apt-get update` and `apt-get install` need the network. The installer now
+treats both as best-effort and continues, because a device typically already
+has `python3`. If they fail and something really is missing, install it while
+online and re-run.
 
 ---
 
@@ -240,6 +304,27 @@ journalctl -u rdm-card-reader -f
 curl http://127.0.0.1:8765/api/card     # tap a card first
 ```
 
+The installer needs no internet: the runtime libraries ship inside the
+source tree (`src/rdm_card_reader/wheels/`).
+
+### Copying the project to the Pi
+
+`install.sh` resolves its source directory from its own location, so copy the
+whole tree (including `src/` — that is where the wheels live) and run it from
+there:
+
+```bash
+# from this machine
+scp -r rdm-card-reader pi@192.168.0.123:~/Desktop/
+
+# on the Pi
+cd ~/Desktop/rdm-card-reader
+sudo ./deploy/install.sh --origin https://your.web.app
+```
+
+If you transferred only a subset of the tree, `--dir` can point the install
+somewhere else, but `src/` must be present or the wheels will be missing.
+
 ---
 
 ## Testing the reader path without a reader