fix: resolve QR code generation silently failing after uv migration

[rramphal]

Description

Replace which segno && segno ... || true with a direct path via
{{ ansible_playbook_python | dirname }}/segno, resolving the binary
from the same venv that Ansible is running in.

Motivation and Context

The QR code task uses which segno && segno ... || true, which relies on
the venv bin/ being on PATH. This works when running via ./algo or
uv run ansible-playbook, but silently produces no QR codes when Ansible
is invoked outside the project venv (e.g., system ansible or
uv tool run --from ansible-core).

This is the only task in the playbooks that uses which to locate a
venv-installed binary — everything else goes through Python imports via
ansible_python_interpreter, which is why nothing else breaks outside
the venv.

Using {{ ansible_playbook_python | dirname }}/segno resolves the binary
directly from the venv, removing the PATH dependency. || true is also
removed — segno is a declared dependency in pyproject.toml, so a failure
should be surfaced, not swallowed.

How Has This Been Tested?

• Verified segno generates valid PNGs when invoked via the dirname path
  approach, including with multiline WireGuard config content
• Linting passes (ansible-lint, yamllint)
• Not tested with a full deployment — would appreciate a deploy test from
  a maintainer before merging

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• I have read the CONTRIBUTING document.
• My code passes all linters (./scripts/lint.sh)
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.
• Dependencies use exact versions (e.g., ==1.2.3 not >=1.2.0).

[CLAassistant]

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