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

bin/quak.ts

@@ -130,9 +130,13 @@ program
     )
     .argument("<dir>", "Output directory")
     .option("--json", "Print result as JSON instead of human-readable summary")
+    .option(
+        "--verify",
+        "Re-hash stored originals and download again any that do not match",
+    )
     // A backup usually runs from cron with nobody watching, so every request
     // it makes retries for longer than the other commands' requests do.
-    .action((dir: string, opts: { json?: boolean }) =>
+    .action((dir: string, opts: { json?: boolean; verify?: boolean }) =>
         run(
             backupCommand(
                 {