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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