Stored by the triggers of notification-service.ts, and read back by
getNotificationsForUser.
id: string
user_id: string
type: string
A NotificationType, not checked by the guard.
data: string
The NotificationData as JSON, not checked by the guard.
read: number
0 or 1.
created_at: string
source_key: string | null
Names the event that caused the notification. A unique index keeps a
second row with the same (user_id, source_key) out; null allows
duplicates.