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/library/read.ts

@@ -80,7 +80,8 @@ export class Photo {
     }
 
     // Fetch and cache the full-resolution original, returning its on-disk path
-    // and byte length. Served from the cache (or the backup download directory)
+    // and byte length; for a live photo, its image's, and its video's path as
+    // `videoPath`. Served from the cache (or the backup download directory)
     // when already present, otherwise fetched through the content pool.
     async original(opts?: ContentOptions): Promise<ContentResult> {
         return this.contentOrThrow().original(this.rec.fileID, opts);