Count each file, byte and upload once in backup statistics (closes #225)

The scanner added a changed file's bytes again for each new chunk and
counted a file as unchanged for each chunk already stored. It now
counts files and bytes once, in the scan phase, and counts its own
uploads, so a --cron run, which has no progress reporter, records
them. The blob count no longer adds earlier paths' blobs again.

The snapshots row now stores the size of all files in total_size and
the referenced blobs' sizes in blob_size, blob_uncompressed_size and
compression_ratio, as docs/DATAMODEL.md says. Those sizes come from
one query, and a failed query fails the snapshot. DATAMODEL.md now
says chunk_count and blob_count count what the run added.

Removed UpdateSnapshotStats and GetCountBySnapshot, which nothing
calls any more.

Model: opus-5-5

TODO.md

@@ -22,6 +22,19 @@ the tag exists and is exercised; what is left is merging `next` to
 
 # Completed Steps
 
+- 2026-10-06: Made the backup summary and the `snapshots` row count each
+  file, byte and upload once
+  ([issue #225](https://git.eeqj.de/sneak/vaultik/issues/225)). The
+  scanner added a file's bytes again for each new chunk and counted a
+  file as unchanged for each chunk already stored, so a first backup
+  reported twice its size and "backed up" could go negative. Upload
+  figures came from the progress reporter, which `--cron` turns off, and
+  `blob_count` counted earlier paths' blobs again for each later path.
+  The scanner now counts uploads itself; `blob_size`,
+  `blob_uncompressed_size` and `compression_ratio` describe the blobs
+  the snapshot references, and `docs/DATAMODEL.md` now says
+  `chunk_count` and `blob_count` count what the run added.
+
 - 2026-10-06: Made command output follow the README's stdout and stderr
   rules ([issue #224](https://git.eeqj.de/sneak/vaultik/issues/224)). The
   startup banner went to stdout, so a `completion` script or a