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>

services/bff/stryker-config.json

@@ -0,0 +1,16 @@
+{
+  "stryker-config": {
+    "solution": "Bff.slnx",
+    "test-projects": ["Bff.Tests/Bff.Tests.csproj"],
+    "reporters": ["progress", "html"],
+    "mutate": [
+      "!**/Program.cs",
+      "!**/DownstreamClients.cs"
+    ],
+    "thresholds": {
+      "high": 95,
+      "low": 90,
+      "break": 90
+    }
+  }
+}