feat(backend): Build escrow audit log service

[KuchiMercy]

Description

Audit log stubs exist but no implementation. Build comprehensive audit trail for all escrow state changes.

Existing: admin-audit-log controller/service stubs | Used by: F-10, F-14

Acceptance Criteria

• AuditLog entity: id, entityType, entityId, action, actorId, actorRole, previousState (JSON), newState (JSON), ipAddress, userAgent, metadata, createdAt
• Record all escrow state changes: created, funded, milestone updated/approved, condition fulfilled, disputed, resolved, refunded, cancelled, completed
• Record admin actions: account freeze, role change, dispute resolution
• previousState/newState as JSON diffs
• AuditLogService.log(action, entity, actor, metadata) callable from any module
• GET /admin/audit-logs with filters
• Pagination (50 per page)
• GET /escrows/:id/audit-log for escrow-specific history
• Non-blocking async writes
• Immutable logs (no UPDATE/DELETE)
• Indexes on entityId, action, createdAt
• Unit tests

Points: 150

[Caneryy]

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

Hi, can I work on this issue?

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Caneryy to this issue.

[drips-wave]

Congratulations, @Adesam007-pr3dator! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

🧑‍💻 @Adesam007-pr3dator: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

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