Narrow ConfigSettings.model_config.env_file to (.env,) — the original
tuple included .env.example (template, should not be a runtime source)
and a non-existent .env.dev. Missing/incomplete .env now fails with a
clear ValidationError instead of being masked by example placeholders.

Remove the unused delay_minutes_refresh_session field from
ConfigSettings. It had no readers and was unreachable via env anyway
because env_nested_delimiter="_" splits on the underscores in the
field name.

tests/conftest.py now seeds the minimal APP__* env vars via
os.environ.setdefault BEFORE src.settings is first imported, so tests
are deterministic and independent of the developer's local .env state.

pytest: 52 passed.

NOTE FOR DEV ENVIRONMENTS: a local .env that still uses the legacy
parser_config__* keys (no APP__ prefix) will now leave required
APP__* fields unset and break consumer.py startup. Migrate .env to
the new schema (see .env.example for the canonical layout).

CHANGELOG updated.