docs: update changelog, CLI reference, user guide, and elevation notes

- CHANGELOG: document hysteresis elevation fix and DEM recalculation feature
- docs/reference/cli.md: add --dem-url to bincio edit and bincio serve tables
- docs/user-guide.md: document "Recalculate from terrain map" button in edit drawer
- docs/elevation.md: mark both short-term and medium-term fixes as implemented

docs/user-guide.md

@@ -73,6 +73,21 @@ Click **Edit** on any activity to:
 
 Changes save instantly. The site rebuilds in the background.
 
+### Recalculating elevation from terrain data
+
+If an activity shows an unrealistic elevation gain (common with GPS-only devices on flat
+routes, or with older Garmin/Wahoo files), the edit drawer has a
+**"Recalculate from terrain map (DEM)"** button.
+
+Clicking it replaces the recorded GPS altitude with SRTM terrain data from the
+[Open-Elevation API](https://open-elevation.com) and recomputes the gain and loss. The
+elevation chart and the summary stats both update. This usually brings the numbers in
+line with what Strava or your device's app reports.
+
+> **Note:** The correction requires a GPS track (activities marked *No GPS* cannot be
+> corrected). The DEM has ~30 m horizontal resolution, so very short or indoor activities
+> are not meaningfully improved.
+
 ### Photo gallery
 
 Upload photos for an activity. They appear in a lightbox on the activity detail page. The server stores them in your data directory.