feat(backend): implement webhook registration, delivery, and retry handling

[amina69]

Summary

This PR adds a complete backend webhook system for user-managed webhooks, including registration, updates, deletion, delivery tracking, retry scheduling, and test ping support.

What changed

• Added webhook registration and management endpoints
• Implemented HMAC-SHA256 signing with the X-Vaultix-Signature header
• Added webhook delivery persistence and delivery history retrieval
• Implemented retry logic with exponential backoff and retry windows
• Disabled webhooks after repeated failures
• Added HTTPS URL validation for production and test ping support
• Added unit tests for webhook service behavior

Notes

• The webhook service now auto-generates a signing secret when one is not provided.
• Webhooks are limited to 10 per user, with a maximum of 8 subscribed events per webhook.

Verification

• TypeScript diagnostics for the changed webhook files report no errors.

closes #417

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[Cedarich]

@amina69

[KuchiMercy]

@amina69 Kindly resolve conflicts