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

ARCHITECTURE.md

@@ -335,10 +335,10 @@ CreateSnapshot(opts)
     │       │
     │       └─► Accumulate statistics
     │
-    ├─► SnapshotManager.UpdateSnapshotStatsExtended()
-    │
     ├─► SnapshotManager.PopulateSnapshotBlobs()   // record referenced blobs
     │
+    ├─► SnapshotManager.UpdateSnapshotStatsExtended()
+    │
     ├─► SnapshotManager.ExportSnapshotMetadata()
     │       │
     │       ├─► Copy database to temp file