fix: photos admin — URL via /api/v1/uploads + static Express

- Nest: servir le volume uploads aussi sous /api/v1/uploads (Traefik /api)
- Flutter: _fullPhotoUrl = base API + chemin /uploads/... (plus d’origine site seul)
- EnfantDossier: tolérer photoUrl camelCase

Made-with: Cursor

frontend/lib/models/dossier_unifie.dart

@@ -203,7 +203,7 @@ class EnfantDossier {
       gender: (json['gender'] ?? json['genre'])?.toString(),
       status: json['status']?.toString(),
       dueDate: json['due_date']?.toString(),
-      photoUrl: json['photo_url']?.toString(),
+      photoUrl: (json['photo_url'] ?? json['photoUrl'])?.toString(),
       consentPhoto: json['consent_photo'] == true,
     );
   }