Add structured log for failed outbound webhook delivery attempts including retry count and next retry time

[Chucks1093]

Summary

When a webhook delivery fails (non-2xx response or timeout), the failure is retried with exponential backoff but no log is emitted per attempt. A warn-level log on each failed attempt would help operators track delivery reliability and identify stuck webhook subscriptions.

Scope

• Emit a warn-level log on each failed delivery attempt with fields: webhook_id, event_type, attempt_number, status_code (null on timeout), next_retry_at, failed_at
• next_retry_at should be null on the final failed attempt (no more retries)
• Log level should escalate to error on the final failed attempt
• Do not log the webhook payload body

Acceptance Criteria

• Warn log on each failed attempt with all six fields
• next_retry_at null on final attempt
• Final attempt logged at error level
• Webhook payload body not present in any log
• attempt_number increments correctly across retries

ETA: 12 hours

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