feat: track notification read receipts per user

- New notification_reads table (migration 008) with unique (notification_id, user_id)
- Notification model: markAsRead inserts into notification_reads; getForUser JOINs
  to show per-user read status; getUnreadCount checks NOT EXISTS in reads table
- getAll() includes read_count subquery for admin view
- Admin view shows 'X / Y' read count for broadcast notifications,
  'Read'/'Unread' badge for user-targeted ones
- Migrates existing read notifications into notification_reads
- AGENTS.md updated with new conventions

database/migrations/008_notification_reads.sql

@@ -0,0 +1,18 @@
+CREATE TABLE IF NOT EXISTS `notification_reads` (
+    `id`              INT UNSIGNED NOT NULL AUTO_INCREMENT,
+    `notification_id` INT UNSIGNED NOT NULL,
+    `user_id`         INT UNSIGNED NOT NULL,
+    `read_at`         DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    PRIMARY KEY (`id`),
+    UNIQUE KEY `idx_notification_user` (`notification_id`, `user_id`),
+    KEY `idx_notification_id` (`notification_id`),
+    KEY `idx_user_id` (`user_id`)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;
+
+-- Migrate existing read receipts for user-targeted notifications
+INSERT IGNORE INTO `notification_reads` (`notification_id`, `user_id`, `read_at`)
+SELECT n.id, u.id, COALESCE(n.read_at, n.created_at)
+FROM notifications n
+JOIN users u ON u.id = n.user_id AND u.status = 'active'
+WHERE n.is_read = 1
+  AND n.user_id IS NOT NULL;