merge: three-leg trust chain contract

docs/three-leg-trust-chain.md

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+# Three-leg trust chain
+
+| Hop | 발급자/행위자 | 검증자 | 핵심 검증 |
+|---|---|---|---|
+| 1 | Google/mock IdP | Keycloak broker | signature, upstream issuer, client audience, expiry, nonce |
+| 2 | Keycloak | SPA | state, PKCE code binding, Keycloak issuer, token response |
+| 3 | Keycloak access token | Spring Resource Server | Keycloak signature/JWKS, issuer, expiry, API audience, roles |
+
+Keycloak은 upstream ID token을 그대로 SPA에 전달하지 않는다. upstream
+identity를 검증·연결한 뒤 자신의 realm key로 새 token을 발급한다. 그 결과
+backend의 trust store와 issuer 설정은 Google 로그인 여부와 무관하게 하나다.
+
+`verify-three-leg-trust-chain.sh`는 두 realm의 discovery issuer가 서로 다르고,
+Spring 코드가 main Keycloak issuer만 신뢰하는지 확인한다. 실제 hop 전체는
+`verify-federation-spa-zero-change.sh`로 검증한다.

scripts/verify-three-leg-trust-chain.sh

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+#!/usr/bin/env sh
+set -eu
+
+base="${KEYCLOAK_URL:-http://localhost:8080}"
+main_issuer="$base/realms/keycloak-patterns"
+mock_issuer="$base/realms/mock-google"
+
+test "$(curl -fsS "$main_issuer/.well-known/openid-configuration" | jq -r .issuer)" \
+  = "$main_issuer"
+test "$(curl -fsS "$mock_issuer/.well-known/openid-configuration" | jq -r .issuer)" \
+  = "$mock_issuer"
+
+rg -Fq 'SPRING_SECURITY_OAUTH2_RESOURCESERVER_JWT_ISSUER_URI:' docker-compose.yml
+if rg -qi 'accounts\\.google\\.com|realms/mock-google' backend/src/main; then
+  echo "backend must trust only the Keycloak issuer, not the upstream IdP" >&2
+  exit 1
+fi
+
+echo "Three-leg trust anchors verified: upstream -> Keycloak -> Resource Server"