Store live photos as their image and their video (closes #107)

A live photo, which Ente stores as one ZIP, is unpacked as it downloads
into its image and its video, each `<fileID>.<ext>` with its extension
from the ZIP, beside `<fileID>.livephoto.json`, which names the two.
Both are checked against the recorded hash and renamed into place only
when both are complete. A ZIP with a second image or video, or whose
parts come to more than 20 times its size plus 16 MiB, is refused. The
backup and the content cache count a live photo as stored only with both
files, album folders link both, `quak get` writes both, and the content
result gives the video as `videoPath`. A ZIP an earlier version stored
is replaced.

Model: opus-5-5

src/filename.ts

@@ -35,3 +35,7 @@ export const safeExtension = (title: string): string => {
     const ext = extname(title);
     return /^\.[A-Za-z0-9]+$/.test(ext) ? ext : ".bin";
 };
+
+// `name` with its extension, if it has one, replaced by `ext` (".mov").
+export const withExtension = (name: string, ext: string): string =>
+    name.slice(0, name.length - extname(name).length) + ext;