Add Eurostar Plus prices and NR advance fare support

- Eurostar scraper now fetches both Standard and Plus (PLUS class code)
  prices/seats in a single API call; each service dict gains plus_price
  and plus_seats fields
- GWR fares scraper gains fetch_advance() which makes two sets of
  paginated calls (standard advance + first-class advance) and returns
  cheapest per departure; shared _run_pages() generator reduces
  duplication in fetch()
- New /api/advance_fares/<station_crs>/<travel_date> endpoint returns
  advance fares as JSON, cached for 24 hours
- Results page gains NR ticket selector (Walk-on / Std Advance / 1st
  Advance) and Eurostar selector (Standard / Plus); total column is
  JS-computed from the selected combination with cheapest/priciest
  highlighting
- Load advance prices button fetches the API lazily; if advance fares
  are already cached they are embedded in the page and applied on load
  so the button is hidden automatically

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_eurostar_scraper.py

@@ -6,16 +6,25 @@ def _gql_response(journeys: list) -> dict:
     return {'data': {'journeySearch': {'outbound': {'journeys': journeys}}}}
 
 
-def _journey(departs: str, arrives: str, price=None, seats=None, service_name='', carrier='ES') -> dict:
+def _journey(departs: str, arrives: str, price=None, seats=None, service_name='', carrier='ES',
+             plus_price=None, plus_seats=None) -> dict:
+    fares = [{
+        'classOfService': {'code': 'STANDARD'},
+        'prices': {'displayPrice': price},
+        'seats': seats,
+        'legs': [{'serviceName': service_name, 'serviceType': {'code': carrier}}]
+        if service_name else [],
+    }]
+    if plus_price is not None or plus_seats is not None:
+        fares.append({
+            'classOfService': {'code': 'PLUS'},
+            'prices': {'displayPrice': plus_price},
+            'seats': plus_seats,
+            'legs': [],
+        })
     return {
         'timing': {'departureTime': departs, 'arrivalTime': arrives},
-        'fares': [{
-            'classOfService': {'code': 'STANDARD'},
-            'prices': {'displayPrice': price},
-            'seats': seats,
-            'legs': [{'serviceName': service_name, 'serviceType': {'code': carrier}}]
-            if service_name else [],
-        }],
+        'fares': fares,
     }
 
 
@@ -34,6 +43,27 @@ def test_parse_graphql_single_journey():
     assert s['train_number'] == 'ES 9014'
     assert s['price'] == 156.0
     assert s['seats'] == 37
+    assert s['plus_price'] is None
+    assert s['plus_seats'] is None
+
+
+def test_parse_graphql_standard_premier_price():
+    data = _gql_response([_journey('09:31', '12:55', price=156, seats=37, service_name='9014',
+                                    plus_price=220, plus_seats=12)])
+    services = _parse_graphql(data, 'Paris Gare du Nord')
+    assert len(services) == 1
+    s = services[0]
+    assert s['price'] == 156.0
+    assert s['seats'] == 37
+    assert s['plus_price'] == 220.0
+    assert s['plus_seats'] == 12
+
+
+def test_parse_graphql_plus_price_none_when_not_returned():
+    data = _gql_response([_journey('09:31', '12:55', price=156, seats=37)])
+    services = _parse_graphql(data, 'Paris Gare du Nord')
+    assert services[0]['plus_price'] is None
+    assert services[0]['plus_seats'] is None
 
 
 def test_parse_graphql_half_pound_price():