fix: actually pass the new optional env vars into the container

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>

docker-compose.yml

@@ -113,6 +113,10 @@ services:
       OIDC_CLIENT_SECRET_WEB: ${OIDC_CLIENT_SECRET_WEB:?required}
       OIDC_CLIENT_ID_MCP: ${OIDC_CLIENT_ID_MCP:?required}
       OIDC_AUDIENCE: ${OIDC_AUDIENCE:?required}
+      # Optional. This block is an explicit allow-list, not env_file — a var
+      # added to .env but not listed here never reaches the container.
+      OIDC_ISSUER_MCP: ${OIDC_ISSUER_MCP:-}
+      OIDC_AUDIENCE_SCOPE: ${OIDC_AUDIENCE_SCOPE:-}
 
       DATABASE_URL: postgres://${POSTGRES_USER}:${POSTGRES_PASSWORD}@db:5432/${POSTGRES_DB}