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/album_photo.dart

@@ -0,0 +1,100 @@
+import 'dart:convert';
+
+/// Represents a shared photo entry in the trip album.
+///
+/// Stores a remote [url], optional [caption], and the uploader identifier for
+/// basic attribution. Persistence is local/offline via JSON helpers.
+class AlbumPhoto {
+  /// Unique identifier for the photo entry.
+  final String id;
+
+  /// Public or signed URL of the photo.
+  final String url;
+
+  /// Optional caption provided by the user.
+  final String? caption;
+
+  /// Name or ID of the uploader for display.
+  final String? uploadedBy;
+
+  /// Creation timestamp.
+  final DateTime createdAt;
+
+  /// Creates an album photo.
+  const AlbumPhoto({
+    required this.id,
+    required this.url,
+    required this.createdAt,
+    this.caption,
+    this.uploadedBy,
+  });
+
+  /// Convenience builder for a new entry.
+  factory AlbumPhoto.newPhoto({
+    required String id,
+    required String url,
+    String? caption,
+    String? uploadedBy,
+  }) {
+    return AlbumPhoto(
+      id: id,
+      url: url,
+      caption: caption,
+      uploadedBy: uploadedBy,
+      createdAt: DateTime.now().toUtc(),
+    );
+  }
+
+  /// Copy with updates.
+  AlbumPhoto copyWith({
+    String? id,
+    String? url,
+    String? caption,
+    String? uploadedBy,
+    DateTime? createdAt,
+  }) {
+    return AlbumPhoto(
+      id: id ?? this.id,
+      url: url ?? this.url,
+      caption: caption ?? this.caption,
+      uploadedBy: uploadedBy ?? this.uploadedBy,
+      createdAt: createdAt ?? this.createdAt,
+    );
+  }
+
+  /// Serialize to JSON.
+  Map<String, dynamic> toJson() {
+    return {
+      'id': id,
+      'url': url,
+      'caption': caption,
+      'uploadedBy': uploadedBy,
+      'createdAt': createdAt.toIso8601String(),
+    };
+  }
+
+  /// Deserialize from JSON.
+  factory AlbumPhoto.fromJson(Map<String, dynamic> json) {
+    return AlbumPhoto(
+      id: json['id'] as String,
+      url: json['url'] as String,
+      caption: json['caption'] as String?,
+      uploadedBy: json['uploadedBy'] as String?,
+      createdAt: DateTime.parse(json['createdAt'] as String),
+    );
+  }
+
+  /// Encode list to string.
+  static String encodeList(List<AlbumPhoto> photos) {
+    return json.encode(photos.map((p) => p.toJson()).toList());
+  }
+
+  /// Decode list from string.
+  static List<AlbumPhoto> decodeList(String raw) {
+    final decoded = json.decode(raw) as List<dynamic>;
+    return decoded
+        .cast<Map<String, dynamic>>()
+        .map(AlbumPhoto.fromJson)
+        .toList();
+  }
+}