chore: readme 수정

.work/keycloak-four-patterns/final/assets/ap3-bff-architecture/ap3-bff-architecture.mmd

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+%% AP3는 browser session을 BFF-owned Bearer 요청으로 바꾼다
+%% question: AP3에서 OAuth token 없는 브라우저 요청은 어느 경계를 지나 내부 Resource Server Bearer 요청이 되는가?
+flowchart LR
+    subgraph g_browser_session_zone["Browser session boundary"]
+        n0(["Browser"])
+    end
+    subgraph g_server_application_zone["Server-side application path"]
+        n1["Spring BFF"]
+        n2[("Authorized-client store")]
+        n3["Internal Resource Server"]
+    end
+    n0 -->|"AP3_SESSION · /bff/api/me"| n1
+    n1 -->|"현재 authorized client 조회"| n2
+    n1 -->|"downstream Bearer 요청"| n3
+    classDef external stroke-dasharray: 6 4
+    class n0 external