feat(infra): Keycloak with mock DigiD/eHerkenning/eIDAS/medewerker realms (closes #3)

Add infra/keycloak/docker-compose.yml (Keycloak 26.1, dev mode, --import-realm)
with four realms imported at boot (infra/keycloak/realms/*.json): digid,
eherkenning, eidas, medewerker. Each has a public big-portal OIDC client
(standard flow + direct access grants) and test users; protocol mappers inject
the identifying claims (bsn / kvk / eidas_id) and medewerker realm roles.

Add `make keycloak-up/keycloak-smoke/keycloak-down`; keycloak-smoke runs
infra/keycloak/check_realms.py (password-grant login per realm, asserts the
claim). Document credentials in docs/synthetic-data.md and a runbook.

Verified: `make keycloak-smoke` green — all four realms log in and return the
expected claims (bsn 123456782, kvk 12345678, eidas_id FR/NL..., role behandelaar).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

infra/keycloak/docker-compose.yml

@@ -0,0 +1,29 @@
+# Keycloak (S-02) with four pre-seeded realms imported at boot:
+#   digid · eherkenning · eidas · medewerker
+# Dev mode, H2 in-memory store, realm JSONs imported from ./realms.
+#
+#   docker compose -f infra/keycloak/docker-compose.yml up -d
+#   curl -s -o /dev/null -w '%{http_code}\n' \
+#     http://localhost:8180/realms/digid/.well-known/openid-configuration   # -> 200
+#
+# Admin console: http://localhost:8180/  (admin / admin — dev only)
+services:
+  keycloak:
+    image: quay.io/keycloak/keycloak:26.1
+    command: ["start-dev", "--import-realm"]
+    environment:
+      KC_BOOTSTRAP_ADMIN_USERNAME: admin
+      KC_BOOTSTRAP_ADMIN_PASSWORD: admin
+      # Older var names too, harmless on 26.x:
+      KEYCLOAK_ADMIN: admin
+      KEYCLOAK_ADMIN_PASSWORD: admin
+      KC_HEALTH_ENABLED: "true"
+      KC_HTTP_ENABLED: "true"
+    ports:
+      - "8180:8080"
+    volumes:
+      - ./realms:/opt/keycloak/data/import:ro,z
+    networks: [cg]
+
+networks:
+  cg: