feat(ap4): add oauth2-proxy OIDC flow

backend/src/main/java/com/example/keycloakpattern/EdgeIdentityController.java

@@ -0,0 +1,47 @@
+package com.example.keycloakpattern;
+
+import java.util.LinkedHashMap;
+import java.util.Map;
+
+import jakarta.servlet.http.HttpServletRequest;
+
+import org.springframework.http.ResponseEntity;
+import org.springframework.web.bind.annotation.GetMapping;
+import org.springframework.web.bind.annotation.RestController;
+
+@RestController
+public class EdgeIdentityController {
+
+    @GetMapping("/edge/me")
+    ResponseEntity<Map<String, Object>> currentUser(HttpServletRequest request) {
+        String authRequestUser = request.getHeader("X-Auth-Request-User");
+        String forwardedUser = request.getHeader("X-Forwarded-User");
+        String user = hasText(authRequestUser) ? authRequestUser : forwardedUser;
+        if (!hasText(user)) {
+            return ResponseEntity.status(401).body(Map.of(
+                "error",
+                "trusted edge identity header is required"
+            ));
+        }
+
+        Map<String, Object> response = new LinkedHashMap<>();
+        response.put("pattern", "AP4-edge-forward-auth");
+        response.put("user", user);
+        response.put("email", firstNonBlank(
+            request.getHeader("X-Auth-Request-Email"),
+            request.getHeader("X-Forwarded-Email")
+        ));
+        response.put("identityHeader", hasText(authRequestUser)
+            ? "X-Auth-Request-User"
+            : "X-Forwarded-User");
+        return ResponseEntity.ok(response);
+    }
+
+    private static String firstNonBlank(String first, String second) {
+        return hasText(first) ? first : second;
+    }
+
+    private static boolean hasText(String value) {
+        return value != null && !value.isBlank();
+    }
+}