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

test/clip-keywords.test.ts

@@ -96,7 +96,7 @@ const SUNSET: { photo: string; sha256: string; embedding: number[] } =
     );
 
 describe("Ente's Discover categories", () => {
-    it("are the 16 of Ente's photos app, each with a text embedding of length 1", () => {
+    it("are the 16 of Ente's photos app, each with a text embedding of norm 1", () => {
         expect(DISCOVER_CATEGORIES.map((c) => c.title)).toEqual([
             "Identity",
             "Screenshots",