fix(mcp): DNS-Rebinding-Schutz aus (server-to-server+Bearer) + MCP-Dienst expose-only

- FastMCP transport_security: enable_dns_rebinding_protection nur an, wenn
  MCP_ALLOWED_HOSTS gesetzt; sonst aus (sonst HTTP 421 "Invalid Host header" bei
  Aufrufen über nginx/Container-Name). Bearer bleibt die Zugriffskontrolle.
- bp-compliance-mcp: Host-Port-Mapping entfernt (8099 war von bp-core-health
  belegt) → expose-only im breakpilot-network, Routing via nginx (Folgeschritt).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

docker-compose.yml

@@ -156,13 +156,15 @@ services:
     container_name: bp-compliance-mcp
     platform: linux/arm64
     command: ["python", "-m", "compliance.mcp.server"]
+    # Internal-only on the breakpilot network; reached via nginx (follow-up) or by
+    # other containers. No host port (avoids host-port conflicts).
     expose:
       - "8099"
-    ports:
-      - "8099:8099"
     environment:
       MCP_PORT: 8099
       CRA_MCP_TOKEN: ${CRA_MCP_TOKEN:-}
+      # Optional: pin Host allowlist (comma-separated) to enable DNS-rebinding protection.
+      MCP_ALLOWED_HOSTS: ${MCP_ALLOWED_HOSTS:-}
     healthcheck:
       disable: true
     restart: unless-stopped