Complete task 4 backend security architecture docs

Taskmaster/Tasks/task-4-5.md

@@ -1,23 +1,23 @@
 ---
 taskmaster_id: "4.5"
-status: "pending"
+status: "done"
 priority: "high"
 depends_on: ["2"]
 parent_id: "4"
 source: "taskmaster"
-generated_at: "2026-05-24T07:15:25.199Z"
+generated_at: "2026-05-24T07:26:29.052Z"
 ---
 
 # 4.5 - Decide session, passkey, and admin step-up architecture
 
-- [ ] 4.5 - Decide session, passkey, and admin step-up architecture #taskmaster #priority/high #status/pending ⏫ 🆔 tm-4-5 ⛔ tm-2
+- [x] 4.5 - Decide session, passkey, and admin step-up architecture #taskmaster #priority/high #status/done ⏫ 🆔 tm-4-5 ⛔ tm-2
 
 ## Metadata
 
 | Field | Value |
 | --- | --- |
 | Taskmaster ID | 4.5 |
-| Status | pending |
+| Status | done |
 | Priority | high |
 | Dependencies | 2 |
 | Parent | 4 - Define backend security and refactor strategy from latest audit |
@@ -28,6 +28,8 @@ Choose browser session model and high-risk admin authentication requirements.
 
 ## Details
 
+Completed. Produced `09 - Audits/Session and Authentication Architecture Decision.md`.
+
 Decide localStorage versus httpOnly cookies, access/refresh token lifetimes, CSRF strategy, refresh rotation, WebAuthn requirements, OAuth requirements, device/session revocation, and whether payouts/role changes require step-up authentication or two-person approval.
 
 ## Verification