fix Last-Modified mtime overwriting post processor mtime (#7529)

https://github.com/mikf/gallery-dl/issues/7529#issuecomment-2989955455

- split '_mtime' into '_mtime_http' and '_mtime_meta'
- add PathFormat.set_mtime() method

gallery_dl/postprocessor/ugoira.py

@@ -236,9 +236,7 @@ class UgoiraPP(PostProcessor):
             pathfmt.realpath = pathfmt.temppath
         else:
             if self.mtime:
-                mtime = pathfmt.kwdict.get("_mtime")
-                if mtime:
-                    util.set_mtime(pathfmt.realpath, mtime)
+                pathfmt.set_mtime()
             return True
 
     def convert_to_archive(self, pathfmt, tempdir):