feat(ParticipantController, ParticipantModel, ParticipantRoutes, DiaryParticipantsPanel, i18n): implement participant status management and UI updates

- Added functionality to update participant attendance status, allowing for 'excused' and 'cancelled' states.
- Enhanced the participant model to include a default attendance status and validation for status values.
- Updated participant routes to support status updates and integrated new status handling in the participant controller.
- Modified the DiaryParticipantsPanel to visually indicate participant status and added a toggle for changing status.
- Expanded localization files to include new keys for participant status, improving accessibility for users in both English and German.

backend/models/Participant.js

@@ -28,6 +28,14 @@ const Participant = sequelize.define('Participant', {
             key: 'id'
         }
     },
+    attendanceStatus: {
+        type: DataTypes.STRING(32),
+        allowNull: false,
+        defaultValue: 'present',
+        validate: {
+            isIn: [['present', 'excused', 'cancelled']]
+        }
+    },
     groupId: {
         type: DataTypes.INTEGER,
         allowNull: true,