Add 503/403 auth tests for the /train-sender endpoint, matching the pattern
already used for /train and /segtrain. Also surface test_sender_registry.py
in CI (it needs no ML stack) and add pytest-asyncio to the install step.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
_check_training_token previously skipped auth when TRAINING_TOKEN was
empty, allowing unauthenticated requests to reach /train and /segtrain.
Now returns 503 ("Training not configured on this node") when the token
is absent, so missing configuration fails closed rather than open.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>