Set output file mtime to match embedded capture time

Some tools (Synology Photos, for videos) key off filesystem mtime
rather than parsing QuickTime metadata for the capture date, so an
otherwise-correct output file would sort/date wrong there. Stamp the
output's mtime from the same clip whose metadata got embedded
(--metadata-from) after encoding.

concat_live_clips.py

@@ -28,6 +28,7 @@ Requirements:
       detects clip overlap): pip install --user numpy
 """
 import argparse
+import os
 import re
 import subprocess
 import sys
@@ -365,6 +366,17 @@ def main():
 
     print(f"Encoding seamless, deduplicated output -> {output}", file=sys.stderr)
     subprocess.run(cmd, check=True)
+
+    creation_dt = fetch_creation_time(clips[args.metadata_from])
+    if creation_dt is not None:
+        ts = creation_dt.timestamp()
+        os.utime(output, (ts, ts))
+        print(f"Set file mtime to {creation_dt.isoformat()} (matches embedded metadata, "
+              f"so apps that key off filesystem time instead of parsing video metadata "
+              f"still sort/date it correctly)", file=sys.stderr)
+    else:
+        print("warning: couldn't determine source creation time; leaving file mtime as-is", file=sys.stderr)
+
     print(f"Done: {output}", file=sys.stderr)