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`;