Fix segment effort duplicates; auto-scan on segment creation

- detect.py: truncate started_at to seconds so dedup key survives JSON round-trip
- store.py: dedup by (activity_id, iso-started_at) string key, not object equality
- server.py: extract _scan_segment_for_user helper; trigger background scan
  for the creating user's activities when a new segment is saved

bincio/segments/store.py

@@ -172,10 +172,10 @@ def save_efforts(data_dir: Path, handle: str, segment_id: str, efforts: list[Seg
 
 
 def add_effort(data_dir: Path, handle: str, segment_id: str, effort: SegmentEffort) -> None:
-    """Append an effort, replacing any existing effort for the same activity."""
+    """Append an effort, replacing any existing effort with the same activity + start time."""
     efforts = load_efforts(data_dir, handle, segment_id)
-    efforts = [e for e in efforts if e.activity_id != effort.activity_id or
-               e.started_at != effort.started_at]
+    key = (effort.activity_id, _iso(effort.started_at))
+    efforts = [e for e in efforts if (e.activity_id, _iso(e.started_at)) != key]
     efforts.append(effort)
     efforts.sort(key=lambda e: e.started_at, reverse=True)
     save_efforts(data_dir, handle, segment_id, efforts)