Implementation of Notification System

docs/migrations/006b_create_notifications_table.sql

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+-- Migration: create notifications table (corrected: `read_by` nullable)
+CREATE TABLE IF NOT EXISTS `notifications` (
+  `id` BIGINT PRIMARY KEY AUTO_INCREMENT,
+  `user_id` INT DEFAULT NULL,
+  `event` VARCHAR(100) NOT NULL,
+  `sub_feed` VARCHAR(100) DEFAULT NULL,
+  `data` TEXT DEFAULT NULL,
+  `target_url` VARCHAR(1024) DEFAULT NULL,
+  `read_by` TEXT DEFAULT NULL,
+  `time_created` TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
+  INDEX (`user_id`),
+  INDEX (`sub_feed`)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;