Basic message implementation

application/_installation/04-create-table-messages.sql

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+CREATE TABLE IF NOT EXISTS `messages` (
+  `id` int(11) NOT NULL AUTO_INCREMENT,
+  `sender_id` int(11) NOT NULL,
+  `receiver_id` int(11) DEFAULT NULL,
+  `group_type` enum('admins','moderators','all_users') DEFAULT NULL,
+  `subject` varchar(255) NOT NULL,
+  `message` text NOT NULL,
+  `is_read` tinyint(1) NOT NULL DEFAULT 0,
+  `created_at` datetime NOT NULL DEFAULT CURRENT_TIMESTAMP,
+  `updated_at` datetime NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
+  PRIMARY KEY (`id`),
+  KEY `sender_id` (`sender_id`),
+  KEY `receiver_id` (`receiver_id`),
+  KEY `is_read` (`is_read`)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;
+
+-- Foreign key constraints
+ALTER TABLE `messages`
+  ADD CONSTRAINT `messages_ibfk_1` FOREIGN KEY (`sender_id`) REFERENCES `users` (`user_id`) ON DELETE CASCADE,
+  ADD CONSTRAINT `messages_ibfk_2` FOREIGN KEY (`receiver_id`) REFERENCES `users` (`user_id`) ON DELETE CASCADE;