feat: escrow event sourcing with full audit trail

[Clinton6801]

Summary

Closes #284. Implements a dedicated append-only EscrowEventStore
alongside the existing event system, with 15 event types, replay
capability, and enhanced querying endpoints.

New files

• enums/escrow-event-type.enum.ts (15 event types)
• entities/escrow-event-store.entity.ts
• services/escrow-event-store.service.ts
• dto/escrow-event-store.dto.ts
• tests/escrow-event-store.service.spec.ts
• Migration: CreateEscrowEventStore (with indexes +
  idempotency constraint)

New/enhanced endpoints

• GET /escrows/:id/events — pagination + type filter added
• GET /escrows/:id/timeline — human-readable reconstruction
• GET /events — global stream (admin, filterable by actor/type/date)
• POST /admin/escrows/:id/replay-events — state replay +
  inconsistency detection

Acceptance criteria

✅ Append-only (INSERT only, update()/delete() throw 403)
✅ Idempotency key deduplication on (escrowId, idempotencyKey)
✅ Monotonic version per escrow
✅ All 15 event types
✅ Timeline with human-readable descriptions
✅ Replay detects status inconsistencies
✅ Old escrow_events table untouched
✅ All 8 tests pass

closes #284

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