Add support for file uploads using new async upload jobs, add admin section for uploading distance files

lib/services/api_service.dart

@@ -4,6 +4,18 @@ import 'package:http/http.dart' as http;
 typedef TokenProvider = String? Function();
 typedef UnauthorizedHandler = Future<bool> Function();
 
+class MultipartFilePayload {
+  MultipartFilePayload({
+    required this.bytes,
+    required this.filename,
+    this.fieldName,
+  });
+
+  final List<int> bytes;
+  final String filename;
+  final String? fieldName;
+}
+
 class ApiService {
   String _baseUrl;
   final http.Client _client;
@@ -192,6 +204,41 @@ class ApiService {
     return _processResponse(response);
   }
 
+  Future<dynamic> postMultipartFiles(
+    String endpoint, {
+    required List<MultipartFilePayload> files,
+    String fieldName = 'files',
+    Map<String, String>? fields,
+    Map<String, String>? headers,
+    bool includeAuth = true,
+    bool allowRetry = true,
+  }) async {
+    Future<http.Response> send() async {
+      final request = http.MultipartRequest(
+        'POST',
+        Uri.parse('$baseUrl$endpoint'),
+      );
+      request.headers.addAll(_buildHeaders(headers, includeAuth: includeAuth));
+      if (fields != null && fields.isNotEmpty) {
+        request.fields.addAll(fields);
+      }
+      for (final file in files) {
+        request.files.add(
+          http.MultipartFile.fromBytes(
+            file.fieldName ?? fieldName,
+            file.bytes,
+            filename: file.filename,
+          ),
+        );
+      }
+      final streamed = await _client.send(request);
+      return http.Response.fromStream(streamed);
+    }
+
+    final response = await _sendWithRetry(send, allowRetry: allowRetry);
+    return _processResponse(response);
+  }
+
   Future<dynamic> postForm(
     String endpoint,
     Map<String, String> data, {