fix(deploy): detect the actually-live blue/green slot, stop nginx-sync clobbering the upstream

- ci-deploy.sh: read_active_port() now probes both slots on /api/health and
  picks the one that really answers; the upstream file only serves as a
  fallback when zero or both slots respond. A stray 'docker compose up' (or a
  clobbered snippet) can no longer derail the next deploy's cutover.
- nginx-sync.sh: cms_upstream_servers.conf is runtime-owned by ci-deploy.sh;
  only seed it when missing, never overwrite what a deploy wrote. This is the
  root cause of tonight's 502: a nginx-sync run reset the snippet (written to
  green:3003 by the last cutover) back to the dead slot A:3002.
- cms_upstream_servers.conf: restore the fresh-host seed default to slot A.

deployment/proxy/cms_upstream_servers.conf

@@ -1,2 +1,2 @@
 # Default; ci-deploy.sh (blue/green) herschrijft dit bestand bij elke switch.
-server 127.0.0.1:3003;
+server 127.0.0.1:3002;