feat: Add services for managing trip-related data

- Implement EmergencyService for handling emergency contacts per trip.
- Create GuestFlagService to manage guest mode flags for trips.
- Introduce NotificationService with local notification capabilities.
- Add OfflineFlagService for managing offline caching flags.
- Develop PackingService for shared packing lists per trip.
- Implement ReminderService for managing reminders/to-dos per trip.
- Create SosService for dispatching SOS events to a backend.
- Add StorageService with album image upload functionality.
- Implement TransportService for managing transport segments per trip.
- Create TripChecklistService for storing and retrieving trip checklists.
- Add TripDocumentService for persisting trip documents metadata.

test: Add unit tests for new services

- Implement tests for AlbumService, BudgetService, EmergencyService, GuestFlagService, PackingService, ReminderService, SosService, TransportService, TripChecklistService, and TripDocumentService.
- Ensure tests cover adding, loading, deleting, and handling corrupted payloads for each service.

lib/services/sos_service.dart

@@ -0,0 +1,54 @@
+import 'dart:convert';
+import 'package:http/http.dart' as http;
+
+/// Service in charge of dispatching SOS events to a backend endpoint.
+///
+/// The backend is expected to accept POST JSON payloads like:
+/// {
+///   "tripId": "...",
+///   "lat": 0.0,
+///   "lng": 0.0,
+///   "message": "...",
+/// }
+class SosService {
+  /// Base URL of the backend (e.g. https://api.example.com/sos).
+  final String baseUrl;
+
+  /// Optional API key header.
+  final String? apiKey;
+
+  /// Optional injected HTTP client (useful for testing).
+  final http.Client _client;
+
+  /// Creates a new SOS service.
+  SosService({required this.baseUrl, this.apiKey, http.Client? client})
+    : _client = client ?? http.Client();
+
+  /// Sends an SOS event. Returns true on HTTP 200.
+  Future<bool> sendSos({
+    required String tripId,
+    required double lat,
+    required double lng,
+    String message = 'SOS déclenché',
+  }) async {
+    final uri = Uri.parse('$baseUrl/sos');
+    try {
+      final response = await _client.post(
+        uri,
+        headers: {
+          'Content-Type': 'application/json',
+          if (apiKey != null) 'Authorization': 'Bearer $apiKey',
+        },
+        body: json.encode({
+          'tripId': tripId,
+          'lat': lat,
+          'lng': lng,
+          'message': message,
+        }),
+      );
+      return response.statusCode == 200;
+    } catch (_) {
+      return false;
+    }
+  }
+}