feat: implement party acceptance flow and webhook retry mechanism

[SharifIbrahimDev]

🚀 Feature: Party Acceptance Flow & Webhook Retry Mechanism

📜 Description

This PR resolves two major issues in Vaultix:

1. [Issue Implement Party Invitation Acceptance/Rejection Flow #58] Escrow Party Acceptance Flow: Implementing a robust mutual-consent mechanism before escrows can be funded and activated.
2. [Issue Backend: Webhook retry mechanism with exponential backoff #197] Webhook Retry Mechanism: Ensuring durability of outgoing webhooks with exponential backoff and a dead-letter queue.

🛠️ Changes Made

• Escrow State Machine: Escrows can no longer transition from PENDING to ACTIVE unless all invited parties explicitly have an ACCEPTED status.
• Party Acceptance Endpoints: Added endpoints POST /escrows/:id/parties/:partyId/accept and reject.
• Durable Webhooks: Introduced WebhookDelivery entity to track individual delivery attempts, logging response codes, error messages, and next retry times.
• Exponential Backoff: Configured an asynchronous retry queue with an exponential backoff schedule (1s, 2s, 4s, 8s, 16s) up to 5 max attempts.
• Admin Visibility: Added endpoints to view permanently failed webhooks (/admin/failed), manually trigger retries (/admin/deliveries/:id/retry), and check health stats (/health).
• Cron Jobs: Integrated @nestjs/schedule to run a durable sweep for delayed/crashed deliveries.

✅ Verification & Testing

• ✅ Added Unit Tests to verify the acceptance workflow and the EscrowService.fund constraint.
• ✅ Appended an E2E testing block for Party Acceptance inside test/e2e/escrow.e2e-spec.ts.
• ✅ All services inject the required Repositories safely.

📌 Related Issues

Closes #58
Closes #197

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[SharifIbrahimDev]

Hi maintainer,

I've resolved the merge conflicts and pushed the updates.

During the resolution, I noticed that main had already implemented similar party acceptance endpoints (acceptPartyInvitation and rejectPartyInvitation). To keep things clean, I removed the redundant duplicate methods from my branch and integrated the functionality seamlessly with the existing code from main.

I also fixed a minor TypeScript type inference error in webhook.service.ts that would have caused the build to fail.

Everything builds successfully (npm run build) and the test suite is fully passing (npm run test) locally. Let me know if you need any further adjustments before merging! 🚀