/health read the APP_VERSION environment variable directly, but
docker-compose passes App__Version, which binds to the App:Version
configuration key. The variable under that name never existed in the
container, so the endpoint always reported "unknown".
Read App:Version through IConfiguration, the approach AdminSystemController
already used for the same value. The resolution rule (configured version,
else assembly version) moves to a shared BuildMetadata helper rather than
being written twice; AdminSystemController now calls it, so the admin page
and /health cannot drift apart.
Local development is unaffected: nothing sets App:Version there, and the
assembly-version fallback still applies.
Tests pin the configuration KEY, not just the behaviour, including that an
App__Version environment variable binds to App:Version. The original bug
failed silently, so a behavioural test alone would not have caught it.
Verified against a running backend: App__Version=9.9.9-test reports
9.9.9-test; unset reports the assembly version rather than "unknown".
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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