add backend server setup with environment configuration, tile caching, and bus stop fetching functionality

lib/pages/map/tfl/tfl_bus_stop_cache.dart

@@ -26,11 +26,10 @@ class TflBusStopCache {
     }
   }
 
-  // rounds to ~1km grid cells so nearby pans reuse the same cache entry
-  static String keyFor(LatLng center) {
-    final lat = (center.latitude * 100).round() / 100;
-    final lon = (center.longitude * 100).round() / 100;
-    return '$lat,$lon';
+  // rounds bbox corners to ~1km grid so nearby pans reuse the same entry
+  static String keyFor(LatLng sw, LatLng ne) {
+    double r(double v) => (v * 100).round() / 100;
+    return '${r(sw.latitude)},${r(sw.longitude)},${r(ne.latitude)},${r(ne.longitude)}';
   }
 
   static List<BusStop>? peek(String key) {

lib/pages/map/tfl/tfl_bus_stop_service.dart

@@ -1,8 +1,10 @@
+import 'dart:async';
 import 'dart:convert';
 import 'dart:math' as math;
 
 import 'package:http/http.dart' as http;
 import 'package:latlong2/latlong.dart';
+import 'package:rra_app/pages/map/constants.dart';
 
 class BusStop {
   const BusStop({
@@ -17,82 +19,94 @@ class BusStop {
   final String name;
   final LatLng position;
 
-  // the letter on the physical bus stop flag e.g. "A", "ZH"
   final String? stopLetter;
-
-  // may be null for parent stops
   final String? towards;
 }
 
 class TflBusStopService {
   TflBusStopService();
 
-  static const _baseUrl = 'https://api.tfl.gov.uk';
-
-  // max radius the TfL API accepts is 1600m but in practice
-  // we clamp to 800 so we dont get swamped at low zoom
-  static const _maxRadius = 800;
-
-  Future<List<BusStop>> fetchNearby(LatLng center, {int radius = 600}) async {
-    final r = radius.clamp(0, _maxRadius);
-
+  // streams bus stops — emits once from cache, then again from TfL if fresher
+  Stream<List<BusStop>> fetchInRect(LatLng sw, LatLng ne) async* {
     final uri = Uri.parse(
-      '$_baseUrl/StopPoint'
-      '?stopTypes=NaptanPublicBusCoachTram'
-      '&lat=${center.latitude}'
-      '&lon=${center.longitude}'
-      '&radius=$r'
-      '&modes=bus',
+      '$kBackendBaseUrl/stops'
+      '?minLat=${sw.latitude}'
+      '&minLon=${sw.longitude}'
+      '&maxLat=${ne.latitude}'
+      '&maxLon=${ne.longitude}',
     );
 
-    final response = await http.get(uri, headers: {
-      'Accept': 'application/json',
-    });
+    final client = http.Client();
+    try {
+      final request = http.Request('GET', uri);
+      final streamed = await client.send(request);
 
-    if (response.statusCode != 200) {
-      throw Exception('TfL StopPoint request failed (${response.statusCode})');
-    }
+      final buffer = StringBuffer();
 
-    final body = jsonDecode(response.body) as Map<String, dynamic>;
-    final stopPoints = body['stopPoints'];
-    if (stopPoints is! List) return const [];
+      await for (final chunk in streamed.stream.transform(utf8.decoder)) {
+        buffer.write(chunk);
+        final text = buffer.toString();
 
-    final stops = <BusStop>[];
-    for (final raw in stopPoints) {
-      if (raw is! Map<String, dynamic>) continue;
+        // SSE events are delimited by \n\n
+        int idx;
+        var remaining = text;
+        while ((idx = remaining.indexOf('\n\n')) != -1) {
+          final event = remaining.substring(0, idx).trim();
+          remaining = remaining.substring(idx + 2);
 
-      final lat = (raw['lat'] as num?)?.toDouble();
-      final lon = (raw['lon'] as num?)?.toDouble();
-      if (lat == null || lon == null) continue;
-
-      final id = raw['id'] as String? ?? '';
-      final name = raw['commonName'] as String? ?? '';
-      final stopLetter = raw['stopLetter'] as String?;
-
-      String? towards;
-      final addInfo = raw['additionalProperties'];
-      if (addInfo is List) {
-        for (final prop in addInfo) {
-          if (prop is Map && prop['key'] == 'Towards') {
-            towards = prop['value'] as String?;
-            break;
+          if (event.startsWith('data: ')) {
+            final json = event.substring(6);
+            final stops = _parseStops(json);
+            if (stops != null) yield stops;
           }
         }
+
+        buffer.clear();
+        buffer.write(remaining);
       }
-
-      stops.add(BusStop(
-        id: id,
-        name: name,
-        position: LatLng(lat, lon),
-        stopLetter: stopLetter,
-        towards: towards,
-      ));
+    } finally {
+      client.close();
+    }
+  }
+
+  static List<BusStop>? _parseStops(String json) {
+    try {
+      final body = jsonDecode(json) as Map<String, dynamic>;
+      final stopPoints = body['stopPoints'];
+      if (stopPoints is! List) return null;
+
+      final stops = <BusStop>[];
+      for (final raw in stopPoints) {
+        if (raw is! Map<String, dynamic>) continue;
+        final lat = (raw['lat'] as num?)?.toDouble();
+        final lon = (raw['lon'] as num?)?.toDouble();
+        if (lat == null || lon == null) continue;
+
+        String? towards;
+        final addInfo = raw['additionalProperties'];
+        if (addInfo is List) {
+          for (final prop in addInfo) {
+            if (prop is Map && prop['key'] == 'Towards') {
+              towards = prop['value'] as String?;
+              break;
+            }
+          }
+        }
+
+        stops.add(BusStop(
+          id: raw['id'] as String? ?? '',
+          name: raw['commonName'] as String? ?? '',
+          position: LatLng(lat, lon),
+          stopLetter: raw['stopLetter'] as String?,
+          towards: towards,
+        ));
+      }
+      return stops;
+    } catch (_) {
+      return null;
     }
-
-    return stops;
   }
 
-  // rough haversine distance in metres, good enough for radius estimation
   static double metersPerPixel(double lat, double zoom) {
     const earthCircumference = 40075016.686;
     final latRad = lat * math.pi / 180.0;