Production correct-credential login returned auth_unavailable 503 because
Flask's SECRET_KEY was unset: wrong-password probes stopped at 401 before
CSRF token issuance, while valid credentials reached _csrf_serializer()
and crashed. App factory now rejects absent/short (<32 char) SECRET_KEY at
startup, and docker_entrypoint.sh fails fast before migration/services.
Bootstrap no longer passes ADMIN_PASSWORD in process arguments; env-only.
Tests: app-factory + entrypoint regression (5 focused passed), full
backend suite 204 passed. Independent review PASS.
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