feat(admin): add webhook delivery monitor endpoint

[Oyinkans0la12]

Closes #474

Summary

Add an admin-only webhook monitoring endpoint that exposes recent failed deliveries, current DLQ depth, and per-subscription delivery statistics.

Changes

• Added GET /api/admin/webhooks/monitor
• Added webhookMonitor service
• Returned the last 100 failed deliveries
• Added current DLQ depth reporting
• Added per-subscription statistics
• Protected the endpoint with existing admin authorization
• Added focused test coverage

Security

• Endpoint is restricted to administrators.
• Uses the project's existing authentication and authorization middleware.
• Returns operational monitoring data only and avoids exposing sensitive webhook information.

Testing

Covered:

• Admin access
• Unauthorized access
• Last-100 failure limit
• Accurate DLQ depth
• Per-subscription statistics
• Empty dataset
• Standardized error responses

Validation

Executed the repository's standard linting and test suite successfully.

Scope

This PR intentionally focuses only on adding the webhook monitoring endpoint and does not redesign webhook delivery, retry behavior, DLQ implementation, authentication, or monitoring architecture.

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[GBOYEE]

@GBOYEE has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Implement following existing patterns and conventions. Add tests, ensure CI passes, and document any new API surface.

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