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/mtime.py

@@ -36,7 +36,7 @@ class MtimePP(PostProcessor):
         if mtime is None:
             return
 
-        pathfmt.kwdict["_mtime"] = (
+        pathfmt.kwdict["_mtime_meta"] = (
             util.datetime_to_timestamp(mtime)
             if isinstance(mtime, datetime) else
             text.parse_int(mtime)