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/models/trip_document.dart

@@ -0,0 +1,123 @@
+import 'dart:convert';
+
+/// Represents a document attached to a trip (billet, passeport, assurance, etc.).
+///
+/// The model stores a human-friendly [title], a [category] to filter in the UI,
+/// an optional [downloadUrl] when the file is hosted remotely, and an optional
+/// [expiresAt] date for reminders (ex: passeport ou ESTA).
+class TripDocument {
+  /// Unique identifier for the document entry.
+  final String id;
+
+  /// Display name chosen by the user (ex: « Billet retour AF763 »).
+  final String title;
+
+  /// Type/category (ex: `billet`, `passeport`, `assurance`, `hebergement`).
+  final String category;
+
+  /// Optional URL to open/download the document (cloud storage or external).
+  final String? downloadUrl;
+
+  /// Optional local file path when offline-only; kept for future sync.
+  final String? localPath;
+
+  /// Optional expiration date to trigger reminders.
+  final DateTime? expiresAt;
+
+  /// Creation timestamp used for stable ordering.
+  final DateTime createdAt;
+
+  /// Creates a trip document entry.
+  const TripDocument({
+    required this.id,
+    required this.title,
+    required this.category,
+    required this.createdAt,
+    this.downloadUrl,
+    this.localPath,
+    this.expiresAt,
+  });
+
+  /// Builds a new entry with defaults.
+  factory TripDocument.newEntry({
+    required String id,
+    required String title,
+    required String category,
+    String? downloadUrl,
+    String? localPath,
+    DateTime? expiresAt,
+  }) {
+    return TripDocument(
+      id: id,
+      title: title,
+      category: category,
+      downloadUrl: downloadUrl,
+      localPath: localPath,
+      expiresAt: expiresAt,
+      createdAt: DateTime.now().toUtc(),
+    );
+  }
+
+  /// Returns a copy with updated fields.
+  TripDocument copyWith({
+    String? id,
+    String? title,
+    String? category,
+    String? downloadUrl,
+    String? localPath,
+    DateTime? expiresAt,
+    DateTime? createdAt,
+  }) {
+    return TripDocument(
+      id: id ?? this.id,
+      title: title ?? this.title,
+      category: category ?? this.category,
+      downloadUrl: downloadUrl ?? this.downloadUrl,
+      localPath: localPath ?? this.localPath,
+      expiresAt: expiresAt ?? this.expiresAt,
+      createdAt: createdAt ?? this.createdAt,
+    );
+  }
+
+  /// Serializes the entry to JSON for persistence.
+  Map<String, dynamic> toJson() {
+    return {
+      'id': id,
+      'title': title,
+      'category': category,
+      'downloadUrl': downloadUrl,
+      'localPath': localPath,
+      'expiresAt': expiresAt?.toIso8601String(),
+      'createdAt': createdAt.toIso8601String(),
+    };
+  }
+
+  /// Deserializes a trip document from JSON.
+  factory TripDocument.fromJson(Map<String, dynamic> json) {
+    return TripDocument(
+      id: json['id'] as String,
+      title: json['title'] as String,
+      category: json['category'] as String,
+      downloadUrl: json['downloadUrl'] as String?,
+      localPath: json['localPath'] as String?,
+      expiresAt: json['expiresAt'] != null
+          ? DateTime.tryParse(json['expiresAt'] as String)
+          : null,
+      createdAt: DateTime.parse(json['createdAt'] as String),
+    );
+  }
+
+  /// Encodes a list of documents to a JSON string.
+  static String encodeList(List<TripDocument> docs) {
+    return json.encode(docs.map((d) => d.toJson()).toList());
+  }
+
+  /// Decodes a list of documents from a JSON string.
+  static List<TripDocument> decodeList(String raw) {
+    final decoded = json.decode(raw) as List<dynamic>;
+    return decoded
+        .cast<Map<String, dynamic>>()
+        .map(TripDocument.fromJson)
+        .toList();
+  }
+}