feat(Calendar): integrate CalendarEvent model and enhance calendar functionality

- Added CalendarEvent model to the backend, establishing relationships with the Club model for better event management.
- Updated server.js to include calendarEventRoutes, enabling API access for calendar events.
- Enhanced CalendarView.vue to support custom event creation and management, improving user interaction with the calendar.
- Refactored various components to streamline event handling and improve overall user experience in the calendar interface.
- Updated TODO and DEVELOPMENT documentation to reflect new calendar features and architectural decisions.

backend/services/calendarEventService.js

@@ -0,0 +1,59 @@
+import { Op } from 'sequelize';
+import CalendarEvent from '../models/CalendarEvent.js';
+import { checkAccess } from '../utils/userUtils.js';
+import HttpError from '../exceptions/HttpError.js';
+
+class CalendarEventService {
+  async listClubEvents(userToken, clubId, year) {
+    await checkAccess(userToken, clubId);
+    const normalizedYear = this.normalizeYear(year);
+    return await CalendarEvent.findAll({
+      where: {
+        clubId,
+        startDate: { [Op.lte]: `${normalizedYear}-12-31` },
+        endDate: { [Op.gte]: `${normalizedYear}-01-01` },
+      },
+      order: [['startDate', 'ASC'], ['title', 'ASC']],
+    });
+  }
+
+  async createClubEvent(userToken, clubId, payload) {
+    await checkAccess(userToken, clubId);
+    const title = String(payload?.title || '').trim();
+    if (!title) throw new HttpError('Titel fehlt', 400);
+    const startDate = this.normalizeDate(payload?.startDate);
+    const endDate = this.normalizeDate(payload?.endDate || payload?.startDate);
+    if (!startDate || !endDate) throw new HttpError('Ungültiges Datum', 400);
+    if (startDate > endDate) throw new HttpError('Enddatum darf nicht vor dem Startdatum liegen', 400);
+
+    return await CalendarEvent.create({
+      clubId,
+      title,
+      startDate,
+      endDate,
+      category: payload?.category ? String(payload.category).trim().slice(0, 64) : null,
+      notes: payload?.notes ? String(payload.notes).trim() : null,
+    });
+  }
+
+  async deleteClubEvent(userToken, clubId, eventId) {
+    await checkAccess(userToken, clubId);
+    const event = await CalendarEvent.findOne({ where: { id: eventId, clubId } });
+    if (!event) throw new HttpError('Event nicht gefunden', 404);
+    await event.destroy();
+    return { success: true };
+  }
+
+  normalizeYear(year) {
+    const parsed = Number.parseInt(year, 10);
+    if (Number.isInteger(parsed) && parsed >= 2020 && parsed <= 2100) return parsed;
+    return new Date().getFullYear();
+  }
+
+  normalizeDate(date) {
+    const text = String(date || '').slice(0, 10);
+    return /^\d{4}-\d{2}-\d{2}$/.test(text) ? text : null;
+  }
+}
+
+export default new CalendarEventService();