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/metadata-backup.ts

@@ -128,7 +128,8 @@ export const extractImageMetadata = (
 // Read a file's original bytes through the library's content cache and extract
 // its embedded image metadata. The bytes come from `photo.original()` — the
 // same on-disk cache the rest of the library fills — rather than a fresh
-// per-call download to a throwaway temp file.
+// per-call download to a throwaway temp file. For a live photo, its `path` is
+// the image.
 const extractExif = async (
     photo: Photo,
 ): Promise<Record<string, unknown> | undefined> => {