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/serve/server.py

@@ -1900,6 +1900,58 @@ async def recalculate_elevation_hysteresis_endpoint(
         raise HTTPException(422, str(e))
 
 
+@app.post("/api/admin/users/{handle}/recompute-elevation")
+async def admin_recompute_elevation(
+    handle: str,
+    bincio_session: Optional[str] = Cookie(default=None),
+) -> JSONResponse:
+    """Recompute elevation gain/loss for all activities of a user from stored timeseries.
+
+    Skips activities with altitude_source == 'dem' (already DEM-corrected).
+    Applies the leading-zero no-fix fix and source-aware hysteresis.
+    Returns patched/skipped/error counts.
+    """
+    _require_admin(bincio_session)
+    user_dir = _get_data_dir() / handle
+    if not user_dir.is_dir():
+        raise HTTPException(404, f"No data directory for '{handle}'")
+
+    from bincio.extract.dem import recalculate_elevation_hysteresis
+    from bincio.render.merge import merge_one
+
+    patched = skipped = errors = 0
+    acts_dir = user_dir / "activities"
+    for json_path in sorted(acts_dir.glob("*.json")):
+        if json_path.name.endswith(".timeseries.json"):
+            continue
+        activity_id = json_path.stem
+        try:
+            detail = json.loads(json_path.read_text(encoding="utf-8"))
+            if detail.get("altitude_source") == "dem":
+                skipped += 1
+                continue
+            ts_path = acts_dir / f"{activity_id}.timeseries.json"
+            if not ts_path.exists():
+                skipped += 1
+                continue
+            ts = json.loads(ts_path.read_text(encoding="utf-8"))
+            ele_arr = ts.get("elevation_m") or []
+            if not any(e for e in ele_arr if e is not None):
+                skipped += 1
+                continue
+            recalculate_elevation_hysteresis(user_dir, activity_id)
+            merge_one(user_dir, activity_id)
+            patched += 1
+        except Exception as exc:
+            log.warning("recompute-elevation[%s/%s]: %s", handle, activity_id, exc)
+            errors += 1
+
+    if patched > 0:
+        _trigger_rebuild(handle)
+
+    return JSONResponse({"ok": True, "patched": patched, "skipped": skipped, "errors": errors})
+
+
 @app.delete("/api/activity/{activity_id}", response_model=GenericResponse)
 async def delete_activity(
     activity_id: str,