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

views/admin/notifications.php

@@ -1,6 +1,7 @@
 <?php
 $notifications = $notifications ?? [];
 $users = $users ?? [];
+$totalActiveUsers = $totalActiveUsers ?? 0;
 $data = $notifications['data'] ?? [];
 ?>
 
@@ -27,7 +28,7 @@ $data = $notifications['data'] ?? [];
                         <th>Title</th>
                         <th>Message</th>
                         <th>Target</th>
-                        <th>Status</th>
+                        <th>Read by</th>
                     </tr>
                 </thead>
                 <tbody>
@@ -53,9 +54,16 @@ $data = $notifications['data'] ?? [];
                                 <td><?= htmlspecialchars(mb_substr($note['message'] ?? '', 0, 80)) ?><?= mb_strlen($note['message'] ?? '') > 80 ? '...' : '' ?></td>
                                 <td><?= $note['user_id'] ? htmlspecialchars($note['target_username'] ?? "User #{$note['user_id']}") : '<em>All Users</em>' ?></td>
                                 <td>
-                                    <span class="status-badge <?= $note['is_read'] ? 'status-online' : 'status-offline' ?>">
-                                        <?= $note['is_read'] ? 'Read' : 'Unread' ?>
-                                    </span>
+                                    <?php if ($note['user_id']): ?>
+                                        <span class="status-badge <?= ($note['read_count'] ?? 0) > 0 ? 'status-online' : 'status-offline' ?>">
+                                            <?= ($note['read_count'] ?? 0) > 0 ? 'Read' : 'Unread' ?>
+                                        </span>
+                                    <?php else: ?>
+                                        <?php $rc = (int) ($note['read_count'] ?? 0); ?>
+                                        <span class="badge badge-<?= $rc >= $totalActiveUsers ? 'success' : ($rc > 0 ? 'warning' : 'secondary') ?>">
+                                            <?= $rc ?> / <?= $totalActiveUsers ?>
+                                        </span>
+                                    <?php endif; ?>
                                 </td>
                             </tr>
                         <?php endforeach; ?>