docs: add latest audit to taskmaster

.taskmaster/tasks/task-4.md

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+# Task 4: Define backend security and refactor strategy from latest audit
+
+Status: pending  
+Priority: high  
+Source audit: `.taskmaster/docs/audit-backend-stack-security-and-refactor-assessment-2026-05-24.md`
+
+Convert the backend stack security/refactor assessment into concrete architecture decisions, documentation deliverables, and developer handoff criteria.
+
+This is an advisory/architecture task. It should run in parallel with immediate backend hardening rather than block urgent remediation.
+
+Subtasks:
+
+1. Assign security ownership and launch decision criteria.
+2. Produce threat model for escrow platform.
+3. Specify funds ledger and escrow state machine.
+4. Create authorization matrix for REST and Socket.IO.
+5. Decide session, passkey, and admin step-up architecture.
+6. Specify webhook security and provider adapter contracts.
+7. Define secure build and supply-chain policy.
+8. Make backend-core stack decision.
+9. Create migration and operational runbooks.