feat(notifications): unified in-app + email dispatch for escrow events

[Rufai-Ahmed]

What

Unified notification layer covering the four escrow events: milestone submitted, funds released, dispute opened, wallet activity.

How

• lib/notifications.ts dispatches each event through both channels: a persisted in-app notification and an email.
• Email sits behind a transport interface in lib/email.ts: Resend over plain fetch when RESEND_API_KEY is set, a console transport otherwise so development works with no provider configured. The provider call is bounded by a 10s abort signal so a hung provider cannot stall a route response.
• Each channel fails independently and only logs, so route handlers can notify after their main side effect without risking the request.
• Every NotificationType must have a message template, so adding an event type forces a template.
• Dispute notifications go to every contract party except whoever raised the dispute.

Testing

15/15 notification dispatch tests (new). Full suite green apart from 5 pre-existing failures in reviews.test.ts, identical on main.

Closes #54