fix: auto-update asset lat/lng on check-in so map shows pins

Previously assets never had coordinates populated - only checkins did.
Now when a checkin is created with GPS data, the asset's latitude/longitude
fields are auto-updated. Also backfilled existing assets from their latest
checkin history. This fixes empty maps — loadAssetPins() filters by
a.latitude != null.

server.py

@@ -1106,6 +1106,14 @@ def create_checkin(body: CheckinCreate):
     conn.commit()
     checkin_id = cursor.lastrowid
 
+    # Auto-update asset's last known location
+    if body.latitude is not None and body.longitude is not None:
+        conn.execute(
+            "UPDATE assets SET latitude = ?, longitude = ? WHERE id = ?",
+            (body.latitude, body.longitude, body.asset_id),
+        )
+        conn.commit()
+
     # Auto-log visit
     row = conn.execute("SELECT created_at FROM checkins WHERE id = ?", (checkin_id,)).fetchone()
     _auto_log_visit(conn, body.user_id, body.asset_id, row["created_at"])