Return errors instead of exiting on rollback, rows close and blob temp file cleanup (closes #345)

WithReadTx ended the process when its rollback failed, including after
Ctrl-C, when database/sql has already rolled the transaction back and
Rollback returns sql.ErrTxDone. It now handles the rollback as WithTx
does. The query functions ended the process when closing their rows
failed; they now ignore the deferred close, because database/sql reports
a close failure through rows.Err(), which each returns, once the rows
are read to the end. The unused database.Fatalf is removed.
cleanupBlobTempFile returns its close or remove failure and the upload
step returns it, so such a failure now fails the backup.

No test makes a rows close fail: SQLite fails it only with the error the
last read already returned.

Model: opus-5-5

internal/database/blob_chunks.go

@@ -58,12 +58,7 @@ func (r *BlobChunkRepository) GetByBlobID(
 		return nil, fmt.Errorf("querying blob chunks: %w", err)
 	}
 
-	defer func() {
-		err := rows.Close()
-		if err != nil {
-			Fatalf("failed to close rows: %v", err)
-		}
-	}()
+	defer func() { _ = rows.Close() }()
 
 	var blobChunks []*BlobChunk