refactor: drop is_read and read_at from notifications table

- is_read and read_at columns are now obsolete since notification_reads
  tracks per-user read receipts
- Removed UPDATE statements in markAsRead/markAllAsRead that touched
  the old columns
- API response still includes computed is_read field via SQL CASE in
  getForUser() JOIN with notification_reads, maintaining Android
  compatibility
- Migration 009 drops both columns

database/migrations/009_remove_notification_obsolete_columns.sql

@@ -0,0 +1,3 @@
+ALTER TABLE `notifications`
+    DROP COLUMN `is_read`,
+    DROP COLUMN `read_at`;

src/Models/Notification.php

@@ -86,13 +86,6 @@ class Notification
         } catch (\Throwable $e) {
             // Already read, ignore
         }
-
-        $this->db->update(
-            'notifications',
-            ['is_read' => 1, 'read_at' => date('Y-m-d H:i:s')],
-            'id = ? AND (user_id IS NULL OR user_id = ?)',
-            [$id, $userId]
-        );
     }
 
     public function markAllAsRead(int $userId): void
@@ -119,13 +112,6 @@ class Notification
                 // Duplicate entry — already read, skip
             }
         }
-
-        $this->db->update(
-            'notifications',
-            ['is_read' => 1, 'read_at' => $now],
-            '(user_id IS NULL OR user_id = ?) AND is_read = 0',
-            [$userId]
-        );
     }
 
     public function getAll(int $page = 1, int $perPage = 20): array