Add training times management and enhance diary view with group selection dialog

This commit introduces the `TrainingTime` model and related functionality, allowing for the management of training times associated with training groups. The backend is updated to include new routes for training times, while the frontend is enhanced with a new dialog in the `DiaryView` for selecting training groups and suggesting available training times. This improves user experience by streamlining the process of scheduling training sessions and managing associated data.

backend/migrations/create_training_times_table.sql

@@ -0,0 +1,19 @@
+-- Migration: Create training_times table
+-- Date: 2025-01-16
+-- For MariaDB/MySQL
+-- Stores training times for training groups
+
+CREATE TABLE IF NOT EXISTS `training_times` (
+  `id` INT(11) NOT NULL AUTO_INCREMENT,
+  `training_group_id` INT(11) NOT NULL,
+  `weekday` TINYINT(1) NOT NULL COMMENT '0 = Sunday, 1 = Monday, ..., 6 = Saturday',
+  `start_time` TIME NOT NULL,
+  `end_time` TIME NOT NULL,
+  `sort_order` INT(11) NOT NULL DEFAULT 0 COMMENT 'Order for displaying multiple times on the same weekday',
+  `created_at` DATETIME NOT NULL,
+  `updated_at` DATETIME NOT NULL,
+  PRIMARY KEY (`id`),
+  KEY `training_group_id` (`training_group_id`),
+  CONSTRAINT `training_times_ibfk_1` FOREIGN KEY (`training_group_id`) REFERENCES `training_group` (`id`) ON DELETE CASCADE ON UPDATE CASCADE
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;
+