feat(notifications): Notification priority & grouping system (#122)

[dagangtj]

Summary

Intelligent notification management with priority levels and smart grouping to reduce noise.

Closes #122

Changes

• Notification service: packages/backend/app/services/notifications.py

  • 5 priority levels: critical, high, medium, low, info
  • 9 categories: bill_due, bill_overdue, budget_warning, large_expense, etc.
  • Auto-priority: category → priority mapping (e.g. bill_overdue → critical)
  • Smart grouping: bills, budget, spending batched together
  • Group priority escalation (highest individual = group priority)
  • Mark read (individual or bulk)
  • Per-user stats

• API: packages/backend/app/routes/notification.py

  • GET /notifications/ — unread with optional grouping
  • POST /notifications/ — create notification
  • POST /notifications/read — mark as read
  • GET /notifications/stats — unread counts by priority

• Tests: 20 test cases

Acceptance Criteria

• Production ready implementation
• Includes tests (20 cases)
• Follows existing code patterns