diff: check double bookings against local store, not remote API

Remote get_reservations() was adding ~3.7s to the diff via redundant
network calls that bypassed the calendar cache. Local store data is
sufficient and consistent with what the UI already displays.

room_cal/actions/diff.py

@@ -60,7 +60,7 @@ async def diff(
     _timed('prices',            _diff_prices(start_date, end_date, api),  timings),
     _timed('closed',            _diff_closed(start_date, end_date, api),  timings),
     _timed('reservations',      _diff_reservations(start_date, end_date, api), timings),
-    _timed('double_bookings',   _diff_double_bookings(start_date, end_date, api), timings),
+    _timed('double_bookings',   _diff_double_bookings(start_date, end_date), timings),
     _timed('user_cal_closed',   _diff_user_cal_closed(start_date, end_date),   timings),
     _timed('ical_feeds_closed', _diff_ical_feeds_closed(start_date, end_date), timings),
     _timed('rule_discounts',    _diff_rule_discounts(start_date, end_date),    timings),
@@ -251,16 +251,29 @@ async def _diff_reservations(
 async def _diff_double_bookings(
   start_date: date,
   end_date: date,
-  api: 'ServiceAPI',
 ) -> list[dict]:
-  """Service API reservations that overlap in date range (double bookings).
+  """Local reservation store entries that overlap in date range (double bookings).
 
   Each row: {'a': {id, guest_name, service, checkin_date, checkout_date},
              'b': {id, guest_name, service, checkin_date, checkout_date},
              'overlap_start', 'overlap_end'}
   Sorted by overlap_start.
   """
-  reservations = await api.get_reservations(start_date, end_date)
+  res_cal = Reservation()
+  cal_events = await res_cal.list(start_date, end_date)
+  reservations = []
+  for e in cal_events:
+    if e.start.date() < start_date:
+      continue
+    d = safe_json_loads(e.x_data)
+    if d and d.get('reservation_id'):
+      reservations.append({
+        'id':         d['reservation_id'],
+        'guest_name': d.get('guest_name', ''),
+        'service':    d.get('service', ''),
+        'start_date': e.start.date(),
+        'end_date':   e.end.date(),
+      })
   results: list[dict] = []
   for i, a in enumerate(reservations):
     for b in reservations[i + 1:]: