feat(webhooks): add signed webhook delivery with retries

[Olasunkanmi975]

##closed #599

Summary

Implements reliable webhook delivery for verification results with HMAC request signing, exponential retry behavior, and dead-letter recording for failed deliveries.

Changes

Webhook Delivery

• Added outbound webhook delivery for verification results
• Preserved existing verification flow

Security

• Added HMAC-SHA256 request signing
• Configurable webhook secret
• Signature included in outbound request headers
• Secrets excluded from logs

Reliability

• Implemented exponential backoff retry policy
• Configurable maximum retry attempts
• Retries limited to retryable failures

Dead-Letter Handling

• Persisted failed deliveries after maximum retry attempts
• Recorded payload, endpoint, attempt count, timestamp, and failure reason
• Prepared failed records for future replay

Testing

Covered:

• HMAC signature generation
• Successful delivery
• Retry behavior
• Exponential backoff
• Maximum retry attempts
• Dead-letter persistence
• Retryable vs non-retryable failures
• Signature header verification

Behavior

No changes to existing verification logic beyond reliable webhook delivery.

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[Olasunkanmi975]

Review this and approve maintainer

[Olasunkanmi975]

Hi maintainer, please review for approval

[Cedarich]

@Olasunkanmi975

[Olasunkanmi975]

@Cedarich HI MAINTAINER CONFLICTS RESOLVE PLEASE REVIEW MY PR