Retry Booking.com calendar fetch on empty response

With the SQLite backend, diff functions hit the Booking API concurrently
(calendar months + reservations), which occasionally triggers rate limiting
that returns an empty 200 response. Retry once after 1s with a warning log.

room_cal/service_api/_booking.py

@@ -501,7 +501,17 @@ class _BookingClient:
     raw = await self._multipart_post(
       f'{_BASE}/fresa/extranet/inventory/fetch', self._monthly_referer(), body
     )
-    return self._parse_calendar_days(raw)
+    result = self._parse_calendar_days(raw)
+    if not result:
+      _log.warning('Booking.com: empty calendar for %d-%02d; retrying', year, month)
+      await asyncio.sleep(1)
+      raw = await self._multipart_post(
+        f'{_BASE}/fresa/extranet/inventory/fetch', self._monthly_referer(), body
+      )
+      result = self._parse_calendar_days(raw)
+      if not result:
+        _log.warning('Booking.com: still empty calendar for %d-%02d after retry', year, month)
+    return result
 
   async def _get_month_calendar(self, month: int, year: int) -> dict[str, DayInfo]:
     key = (month, year)