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

backend/bin/server.dart

@@ -0,0 +1,52 @@
+import 'dart:io';
+
+import 'package:dotenv/dotenv.dart';
+import 'package:shelf/shelf.dart' show Middleware, Response;
+import 'package:rra_backend/rra_backend.dart';
+import 'package:shelf/shelf.dart';
+import 'package:shelf/shelf_io.dart' as io;
+
+Middleware _logger() => (handler) => (request) async {
+  final sw = Stopwatch()..start();
+  final response = await handler(request);
+  sw.stop();
+  final ms = sw.elapsedMilliseconds;
+  print('${request.method} ${response.statusCode} ${ms}ms ${request.url.path}');
+  return response;
+};
+
+Middleware _cors() => (handler) => (request) async {
+  if (request.method == 'OPTIONS') {
+    return Response.ok('', headers: _corsHeaders);
+  }
+  final response = await handler(request);
+  return response.change(headers: _corsHeaders);
+};
+
+const _corsHeaders = {
+  'Access-Control-Allow-Origin': '*',
+  'Access-Control-Allow-Methods': 'GET, OPTIONS',
+  'Access-Control-Allow-Headers': 'Origin, Content-Type',
+};
+
+void main() async {
+  final env = DotEnv(includePlatformEnvironment: true)..load();
+
+  final mapboxToken = env['MAPBOX_ACCESS_TOKEN'];
+  if (mapboxToken == null || mapboxToken.isEmpty) {
+    stderr.writeln('ERROR: MAPBOX_ACCESS_TOKEN not set');
+    exit(1);
+  }
+
+  final port = int.tryParse(env['PORT'] ?? '8080') ?? 8080;
+
+  await TileCache.init();
+
+  final handler = const Pipeline()
+      .addMiddleware(_logger())
+      .addMiddleware(_cors())
+      .addHandler(buildRouter(mapboxToken).call);
+
+  final server = await io.serve(handler, InternetAddress.anyIPv4, port);
+  print('rra_backend listening on :${server.port}');
+}