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>

Makefile

@@ -12,6 +12,8 @@ OZ_COMPOSE  := infra/openzaak/docker-compose.yml
 OZ_BASE     := http://localhost:8000
 NRC_COMPOSE := infra/opennotificaties/docker-compose.yml
 NRC_BASE    := http://localhost:8001
+KC_COMPOSE  := infra/keycloak/docker-compose.yml
+KC_BASE     := http://localhost:8180
 STACK_FILES := -f $(OZ_COMPOSE) -f $(NRC_COMPOSE)
 
 # On a rootless Podman dev box, point Docker CLI/Compose at the Podman socket —
@@ -24,7 +26,7 @@ export DOCKER_HOST := unix://$(PODMAN_SOCK)
 endif
 endif
 
-.PHONY: ci lint build unit smoke down changelog openzaak-up openzaak-smoke openzaak-seed openzaak-down stack-up stack-smoke stack-down help
+.PHONY: ci lint build unit smoke down changelog openzaak-up openzaak-smoke openzaak-seed openzaak-down stack-up stack-smoke stack-down keycloak-up keycloak-smoke keycloak-down help
 
 ## ci: run the full pipeline — lint, build, unit, smoke (mirrors Gitea Actions)
 ci: lint build unit smoke
@@ -107,6 +109,21 @@ stack-smoke: stack-up
 stack-down:
 	docker compose $(STACK_FILES) down --volumes
 
+## keycloak-up: start Keycloak with the four imported realms
+keycloak-up:
+	docker compose -f $(KC_COMPOSE) up -d
+
+## keycloak-smoke: start Keycloak, then verify each realm logs in + returns its claim
+keycloak-smoke: keycloak-up
+	@bash -c 'for i in $$(seq 1 60); do \
+	    c=$$(curl -s -o /dev/null -w "%{http_code}" $(KC_BASE)/realms/digid/.well-known/openid-configuration || true); \
+	    [ "$$c" = "200" ] && break; sleep 3; done; echo "Keycloak ready ($$c)"'
+	python3 infra/keycloak/check_realms.py
+
+## keycloak-down: stop and remove Keycloak
+keycloak-down:
+	docker compose -f $(KC_COMPOSE) down --volumes
+
 ## help: list available targets
 help:
 	@grep -E '^## ' $(MAKEFILE_LIST) | sed 's/^## //'