feat(notifications): add delivery mechanisms (in-app, email, webhook)

[martsokha]

Context

A notification response type exists but the delivery mechanism is not implemented beyond basic list/mark-as-read.

Requirements

• In-app notifications: delivered via existing API (list, mark-as-read) and optionally via WebSocket for real-time push
• Email notifications: configurable per-user preferences for which event types trigger emails
• Webhook notifications: per-user or per-workspace webhook for notification delivery to external systems
• User preferences API for controlling notification channels and event type subscriptions

References

• Server docs: ARCHITECTURE.md - Data Model