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

internal/snapshot/progress.go

@@ -66,7 +66,6 @@ type ProgressStats struct {
 	BlobsCreated     atomic.Int64
 	BlobsUploaded    atomic.Int64
 	BytesUploaded    atomic.Int64
-	UploadDurationMs atomic.Int64 // Total milliseconds spent uploading
 	CurrentFile      atomic.Value // stores string
 	TotalSize        atomic.Int64 // Total size to process (set after scan phase)
 	TotalFiles       atomic.Int64 // Total files to process in phase 2
@@ -231,9 +230,6 @@ func (pr *ProgressReporter) ReportUploadComplete(
 	// Clear current upload
 	pr.stats.CurrentUpload.Store((*UploadInfo)(nil))
 
-	// Add to total upload duration
-	pr.stats.UploadDurationMs.Add(duration.Milliseconds())
-
 	// Calculate speed
 	if duration < time.Millisecond {
 		duration = time.Millisecond