quak backup --verify re-hashes stored originals and downloads again any that do not match (closes #168)

`--verify`, or `lib.backup({ verify: true })`, hashes each original already
at its save path as the download check does, streamed, a live photo as
`<imageHash>:<videoHash>`. A mismatch is logged, removed and fetched again in
the same run; a failed fetch goes into `failures.json`. A file with no
recorded hash counts as unchecked. The result, the summary and `--json` gain
`verified`, `mismatched` and `unchecked`.

Judgement call: a stored original that cannot be read for hashing is recorded as failed and left in place.
Judgement call: the summary prints the three counts only with `--verify`.
Judgement call: the README's `BackupOptions` list does not name `verify`, to stay clear of #178's edit of that paragraph; Backup layout documents it.

Model: opus-5-5

test/live-photo.ts

@@ -30,7 +30,8 @@ export const livePhotoZip = (
     },
 ): Uint8Array => zipSync(entries);
 
-const blake2b = (bytes: Uint8Array): string =>
+// The content hash Ente's clients record for an original's bytes.
+export const blake2b = (bytes: Uint8Array): string =>
     createHash("blake2b512").update(bytes).digest("base64");
 
 // The hash Ente's clients record for a live photo: the unkeyed BLAKE2b-512 of