ci: use liveness for alert-receiver in the smoke stack

CI has no ALERT_FORWARD_URL, so the receiver reports archive-only mode as
not-ready on /readyz by design (503) and the stack never settles. Probe
/healthz in CI only; production keeps the strict delivery readiness gate.

docker-compose.ci.yml

@@ -117,3 +117,13 @@ services:
       interval: 30s
       timeout: 10s
       retries: 5
+
+  alert-receiver:
+    # CI does not configure ALERT_FORWARD_URL, so the receiver correctly reports
+    # archive-only mode as NOT ready (/readyz = 503). The smoke stack only needs
+    # the process alive; production keeps the strict /readyz delivery gate.
+    healthcheck:
+      test: ["CMD-SHELL", "wget -qO- http://127.0.0.1:8080/healthz >/dev/null"]
+      interval: 30s
+      timeout: 5s
+      retries: 5