Missing Provider Webhook/Callback Specification

[fenar]

Severity: Medium
Affected Services: aex-work-publisher, aex-contract-engine

Problem:
No specification for how providers receive:

1. Work broadcast notifications
2. Contract award notifications
3. Contract status updates

Resolution:
Add webhook specification defining:

• Callback URL registration (in provider-registry)
• Payload formats for each notification type
• Retry policy for failed deliveries
• Signature verification for security