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/snapshot/scanner.go

@@ -1553,19 +1553,23 @@ func (s *Scanner) handleBlobReady(
 	blobExists, err := s.uploadBlobIfNeeded(
 		ctx, blobPath, blobWithReader, startTime, result)
 	if err != nil {
-		s.cleanupBlobTempFile(blobWithReader)
+		cleanupErr := s.cleanupBlobTempFile(blobWithReader)
 
-		return fmt.Errorf("uploading blob %s: %w", finishedBlob.Hash, err)
+		return errors.Join(
+			fmt.Errorf("uploading blob %s: %w", finishedBlob.Hash, err), cleanupErr)
 	}
 
 	err = s.recordBlobMetadata(ctx, finishedBlob, blobExists, startTime)
 	if err != nil {
-		s.cleanupBlobTempFile(blobWithReader)
+		cleanupErr := s.cleanupBlobTempFile(blobWithReader)
 
-		return err
+		return errors.Join(err, cleanupErr)
 	}
 
-	s.cleanupBlobTempFile(blobWithReader)
+	err = s.cleanupBlobTempFile(blobWithReader)
+	if err != nil {
+		return err
+	}
 
 	// Chunks from this blob are now committed to DB - remove from pending set
 	s.removePendingChunkHashes(blobWithReader.InsertedChunkHashes)
@@ -1759,20 +1763,22 @@ func (s *Scanner) recordBlobMetadata(
 }
 
 // cleanupBlobTempFile closes and removes the blob's temporary file
-func (s *Scanner) cleanupBlobTempFile(blobWithReader *blob.WithReader) {
+func (s *Scanner) cleanupBlobTempFile(blobWithReader *blob.WithReader) error {
 	if blobWithReader.TempFile != nil {
 		tempName := blobWithReader.TempFile.Name()
 
 		err := blobWithReader.TempFile.Close()
 		if err != nil {
-			log.Fatal("Failed to close temp file", "file", tempName, "error", err)
+			return fmt.Errorf("closing blob temp file: %w", err)
 		}
 
 		err = s.fs.Remove(tempName)
 		if err != nil {
-			log.Fatal("Failed to remove temp file", "file", tempName, "error", err)
+			return fmt.Errorf("removing blob temp file: %w", err)
 		}
 	}
+
+	return nil
 }
 
 // streamingChunkInfo tracks chunk metadata collected during streaming