Fix elevation gain inflation from device no-fix leading zeros

Apple Watch and similar devices record exactly 0.0 for elevation while
waiting for barometric/GPS lock, then jump to the real altitude. The
hysteresis accumulator was seeding from 0.0, counting the full jump as
ascent. Fix: detect a leading near-zero run followed by a large jump
and seed the accumulator from the first real value instead.

Applied in both _elevation() (fresh extractions) and
recalculate_elevation_hysteresis() (recompute path). Added a bulk
admin endpoint POST /api/admin/users/{handle}/recompute-elevation and
corresponding button to fix existing stored activities.

bincio/extract/dem.py

@@ -351,6 +351,13 @@ def recalculate_elevation_hysteresis(user_dir: Path, activity_id: str) -> dict:
     altitude_source = detail.get("altitude_source", "unknown")
     threshold = 1.0 if altitude_source == "barometric" else 3.0
 
+    # Strip leading no-fix zeros (same logic as metrics._elevation)
+    if elevations and abs(elevations[0]) < 0.5:
+        for i, e in enumerate(elevations):
+            if abs(e) > threshold:
+                elevations = elevations[i:]
+                break
+
     # Pre-smooth to suppress noise, then accumulate with low dead-band
     smoothed = _moving_average(elevations, _MA_WINDOW_S)
     gain, loss = _hysteresis_gain_loss(smoothed, threshold)