Normalize timestamp group folder names

grouping.py

@@ -12,11 +12,23 @@ Files are sorted by their current path and moved into one folder named
 from __future__ import annotations
 
 import os
+import re
 import shutil
 import sys
 from pathlib import Path
 
 
+ORDERED_TIMESTAMP_STEM_RE = re.compile(r"^(?P<timestamp>\d{8}_\d{6})-[1-9]\d*$")
+
+
+def group_stem(path: Path) -> str:
+    stem = path.stem
+    match = ORDERED_TIMESTAMP_STEM_RE.match(stem)
+    if match:
+        return match.group("timestamp")
+    return stem
+
+
 def unique_path(path: Path) -> Path:
     if not path.exists():
         return path
@@ -87,8 +99,8 @@ def group_items(items: list[Path]) -> Path | None:
         return None
 
     sorted_items = sorted(items, key=lambda item: item.name.lower())
-    first = sorted_items[0].stem
-    last = sorted_items[-1].stem
+    first = group_stem(sorted_items[0])
+    last = group_stem(sorted_items[-1])
     target_dir = unique_path(working_dir_for_items(sorted_items) / f"{first}-{last}")
     target_dir.mkdir(parents=True)