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/check_realms.py

@@ -0,0 +1,60 @@
+#!/usr/bin/env python3
+"""Smoke-check the Keycloak realms: each realm's OIDC login works (password grant)
+and returns its expected identifying claim. Stdlib only. Exits non-zero on failure.
+"""
+import base64, json, sys, urllib.error, urllib.parse, urllib.request
+
+BASE = "http://localhost:8180"
+CLIENT = "big-portal"
+PWD = "test123"
+
+# realm, user, claim ("__roles__" => check realm_access.roles), expected-contains
+CHECKS = [
+    ("digid", "jan-burger", "bsn", "123456782"),
+    ("eherkenning", "acme-ondernemer", "kvk", "12345678"),
+    ("eidas", "pierre-dupont", "eidas_id", "FR/NL"),
+    ("medewerker", "merel-behandelaar", "__roles__", "behandelaar"),
+]
+
+
+def decode(jwt):
+    p = jwt.split(".")[1]
+    p += "=" * (-len(p) % 4)
+    return json.loads(base64.urlsafe_b64decode(p))
+
+
+def grant(realm, user):
+    data = urllib.parse.urlencode({
+        "grant_type": "password", "client_id": CLIENT,
+        "username": user, "password": PWD, "scope": "openid",
+    }).encode()
+    req = urllib.request.Request(
+        f"{BASE}/realms/{realm}/protocol/openid-connect/token", data=data,
+        headers={"Content-Type": "application/x-www-form-urlencoded"})
+    with urllib.request.urlopen(req, timeout=20) as r:
+        return json.loads(r.read())
+
+
+def main():
+    ok = True
+    for realm, user, claim, expect in CHECKS:
+        try:
+            at = decode(grant(realm, user)["access_token"])
+            if claim == "__roles__":
+                val = at.get("realm_access", {}).get("roles", [])
+                good = expect in val
+            else:
+                val = at.get(claim)
+                good = val is not None and expect in str(val)
+            print(f"{realm:12} {user:18} login OK | {claim} = {val} "
+                  f"[{'OK' if good else 'UNEXPECTED'}]")
+            ok = ok and good
+        except urllib.error.HTTPError as e:
+            ok = False
+            print(f"{realm:12} {user:18} LOGIN FAILED {e.code}: {e.read()[:200]!r}")
+    print("keycloak smoke OK" if ok else "keycloak smoke FAILED")
+    sys.exit(0 if ok else 1)
+
+
+if __name__ == "__main__":
+    main()