Test ML data with faces and no CLIP embedding, and name the norm

Each backup command's tests now have a file whose ML data holds faces and
no CLIP embedding; its JSON has the ML data and no keywords. The helper
that gives a vector's norm is called norm. The README names what a
video's JSON lacks, and the generator states its download size with the
Python packages counted.

Model: opus-5-5

bin/clip-keyword-vectors

@@ -2,7 +2,8 @@
 # Writes src/clip-keyword-vectors.ts and test/clip-keywords-sunset.json by
 # running bin/clip-keyword-vectors.py in a Python image pinned by digest, with
 # its packages pinned by hash. Run it by hand from a checkout, then `make fmt`;
-# no gate runs it. It downloads about 215 MB, most of it Ente's two CLIP models.
+# no gate runs it. Besides the Python image, it downloads about 265 MB, most of
+# it Ente's two CLIP models.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"