fix(sleep): store sleep timestamps from GMT not Local (displayed +1h in BST)

backend/app/services/garmin_connect_sync.py

@@ -555,10 +555,15 @@ def _parse_day(stats, sleep_data, hrv_data) -> dict:
         _set(row, "sleep_rem_s",      dto.get("remSleepSeconds"))
         _set(row, "sleep_awake_s",    dto.get("awakeSleepSeconds"))
 
-        # Timestamps are milliseconds since epoch in local time
-        for key, col in (("sleepStartTimestampLocal", "sleep_start"),
-                         ("sleepEndTimestampLocal",   "sleep_end")):
-            ms = dto.get(key)
+        # Use the GMT timestamps (true epoch-ms instants). The *Local fields are
+        # the GMT value pre-shifted by the local UTC offset, so storing them as
+        # UTC and letting the frontend convert to local double-applies the offset
+        # (sleep displayed +1h in BST). GMT stored as UTC converts back correctly.
+        for gmt_key, local_key, col in (
+            ("sleepStartTimestampGMT", "sleepStartTimestampLocal", "sleep_start"),
+            ("sleepEndTimestampGMT",   "sleepEndTimestampLocal",   "sleep_end"),
+        ):
+            ms = dto.get(gmt_key) or dto.get(local_key)
             if ms:
                 _set(row, col, datetime.fromtimestamp(ms / 1000, tz=timezone.utc).isoformat())