Notification services

Folder: services/notifications/

File	Role
`fcm-sender.service.ts`	Send Firebase Cloud Message
`notification-templates.ts`	Title/body templates per notification type
`send-due-reminders.service.ts`	Cron: cards/items due today
`send-streak-warnings.service.ts`	Cron: study streak at risk (IST window)
`record-notification-open.service.ts`	Track when user opens a push
`notification-metrics.service.ts`	Admin metrics aggregation

Internal routes trigger senders; user routes manage tokens/prefs.

Dry run: env.NOTIFICATION_DRY_RUN logs instead of sending.