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/migrations/create_calendar_events_table.sql

@@ -0,0 +1,15 @@
+CREATE TABLE IF NOT EXISTS calendar_events (
+  id INT AUTO_INCREMENT PRIMARY KEY,
+  club_id INT NOT NULL,
+  title VARCHAR(255) NOT NULL,
+  start_date DATE NOT NULL,
+  end_date DATE NOT NULL,
+  category VARCHAR(64) NULL,
+  notes TEXT NULL,
+  created_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+  updated_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
+  KEY idx_calendar_events_club_start (club_id, start_date),
+  CONSTRAINT fk_calendar_events_club
+    FOREIGN KEY (club_id) REFERENCES clubs(id)
+    ON DELETE CASCADE
+);