Face-recognition e2e fixtures

The face-recognition e2e tests (tests/e2e-backends/backend_test.go) don't require committed fixture JPEGs. They follow the same pattern as BACKEND_TEST_AUDIO_URL (whisper): the Makefile target passes HTTP URLs via BACKEND_TEST_FACE_IMAGE_*_URL, and the suite downloads them at BeforeAll time.

For the Makefile targets in Makefile, the defaults point at NASA's public-domain astronaut portraits on nasa.gov. NASA images are released into the public domain by U.S. federal work (see https://www.nasa.gov/nasa-brand-center/images-and-media/).

If you want to run the suite fully offline, drop three JPEGs into this directory with the names the Makefile expects and flip the env vars to the _FILE variants:

tests/fixtures/faces/person_a_1.jpg   # person A, photo 1
tests/fixtures/faces/person_a_2.jpg   # person A, photo 2 (different angle/lighting)
tests/fixtures/faces/person_b.jpg     # a different person

The suite asserts relative ordering only (d(a1,a2) < d(a1,b)) — the absolute distance ceiling is set per-model via BACKEND_TEST_VERIFY_DISTANCE_CEILING so SFace (which uses a wider distance distribution than ArcFace) can share the same suite.