fix: refine hysteresis recalculation with MA pre-smoothing and lower thresholds

- dem.py: pre-smooth elevation with 30s moving average before hysteresis
  in recalculate_elevation_hysteresis(); thresholds drop from 5m/10m to
  1m (barometric) / 3m (GPS) — accurate after noise is smoothed out
- dem.py: widen DEM median-filter window 45s → 60s
- dem.py: rename response key source → altitude_source for consistency
- writer.py: write altitude_source into detail JSON at extract time
- tests/test_dem.py: 21 unit tests for pure functions and file-level hysteresis
- tests/test_edit_server.py: 11 TestClient API tests for both recalculate endpoints
- add httpx as dev dependency (required by FastAPI TestClient)

bincio/extract/writer.py

@@ -93,6 +93,7 @@ def write_activity(
         "source": source,
         "source_file": activity.source_file,
         "source_hash": activity.source_hash,
+        "altitude_source": activity.altitude_source,
         "strava_id": activity.strava_id,
         "duplicate_of": duplicate_of,
         "privacy": privacy,