fix: line-buffer sync output so a redirected log shows progress live

A sync runs for hours and is normally watched through a redirected log,
where Python's block buffering withheld the per-source progress lines
until they happened to flush. The gallery-dl subprocesses write to the
same descriptor unbuffered, so the log also interleaved out of order.

Reconfiguring the streams in-process rather than relying on `python3 -u`
means it holds however the script is invoked.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

scripts/gdl-sync.py

@@ -484,6 +484,13 @@ def publish(staging: Path, dest: str, dry_run: bool) -> int:
 
 
 def main() -> int:
+    # A sync runs for hours and is normally watched through a redirected log,
+    # where Python's block buffering would withhold progress until it happened
+    # to flush -- and the gallery-dl subprocesses write to the same descriptor
+    # unbuffered, so the log would also interleave out of order.
+    sys.stdout.reconfigure(line_buffering=True)
+    sys.stderr.reconfigure(line_buffering=True)
+
     ap = argparse.ArgumentParser(description=__doc__,
                                  formatter_class=argparse.RawDescriptionHelpFormatter)
     ap.add_argument("--index", required=True,