Record the stored size of a blob a backup finds already stored (closes #300)

When a finished blob already existed at the destination, the upload was
skipped but the blob row, and so the manifest, kept the size of the
local copy. That copy differs from the stored object when
compression_level or the recipients changed since it was written, so
shallow and deep verify failed on a good snapshot. Marking a blob
uploaded now records the size of the object in storage: for an existing
blob, the size Stat reports.

Judgement call: the test adds a recipient rather than changing the
compression level, because an extra age recipient always makes the
local copy larger, while two zstd levels need not compress the test
data differently.

Model: opus-5-5

TODO.md

@@ -22,6 +22,16 @@ the tag exists and is exercised; what is left is merging `next` to
 
 # Completed Steps
 
+- 2026-10-13: Recorded the size of the stored object for a blob a backup
+  found already stored
+  ([issue #300](https://git.eeqj.de/sneak/vaultik/issues/300)). When the
+  upload was skipped, the blob row and the manifest kept the size of the
+  local copy, which differs from the stored object's when
+  `compression_level` or the recipients changed since it was written.
+  `snapshot verify` then failed on a good snapshot, shallow and deep,
+  and the listed sizes were wrong. Marking the blob uploaded now records
+  the size of the object in storage.
+
 - 2026-10-12: Bound the local index to the destination an `s3.*` config
   names ([issue #298](https://git.eeqj.de/sneak/vaultik/issues/298)).
   Only `storage_url` was checked, so after `s3.bucket` or `s3.prefix`