ci(bff): compose wiring, verify-bff live check, mutation baseline (refs #8)

Wire the bff service in compose (Keycloak authority + downstream domain/projection
URLs, depends_on domain/projection healthy + keycloak started). run-bff-check.sh
verifies the BFF end-to-end against the up stack: 401 without a token, 202 with a
real digid token minted via direct grant against keycloak:8080 (host-consistent
issuer, ADR-0010), and an anonymous public-safe openbaar register (never a bsn).
Wired as verify-bff (Makefile + verify chain + CI step). Stryker baseline for the
BFF's pure logic (OpenbaarProjection) at 100% (break 90); Program/HTTP adapters are
covered by the endpoint tests + verify-bff. CI uploads the bff mutation report.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

infra/docker-compose.yml

@@ -344,6 +344,13 @@ services:
       context: ../services/bff
       dockerfile: Dockerfile
     image: register-referentie/bff:dev
+    environment:
+      # The BFF is the portals' only backend; it validates digid tokens and fans out (ADR-0010).
+      # Keycloak (start-dev) derives the issuer from the request host, so the BFF authority and the
+      # verify token request both use keycloak:8080 to keep the issuer consistent.
+      Keycloak__Authority: http://keycloak:8080/realms/digid
+      Downstream__Domain__BaseUrl: http://domain:8080/
+      Downstream__Projection__BaseUrl: http://projection-api:8080/
     ports:
       - "8080:8080"
     healthcheck:
@@ -352,6 +359,13 @@ services:
       timeout: 3s
       retries: 5
       start_period: 10s
+    depends_on:
+      domain:
+        condition: service_healthy
+      projection-api:
+        condition: service_healthy
+      keycloak:
+        condition: service_started
     networks: [cg]
 
   # ── Read projection (S-06) ────────────────────────────────────────────────