OIDC_ISSUER_MCP and OIDC_AUDIENCE_SCOPE were added to .env and read by the
app, but never reached it: the compose `environment:` block is an explicit
allow-list, not env_file, so anything not named there is silently dropped.
The aud scope only worked because its computed default happened to be right.
Also make optional vars tolerate the empty string. compose renders `${VAR:-}`
as "" rather than omitting the key, so an unset optional var would arrive as
"" and fail .url()/.min(1) validation — taking the app down at boot rather
than falling back to its default.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>