Fix cross-namespace DNS failure calling users service from ai-assistant

NXTGAUGE_USERS_URL used the short service name "nxtgauge-rust-users",
which only resolves within the same namespace. The prod overlay places
ai-assistant in nxtgauge-ai while the users service is in nxtgauge, so
every help-center search and ticket-creation call failed with a DNS
resolution error (surfaced as a 502 to the client). Use the full
in-cluster FQDN instead.

apps/nxtgauge-ai-assistant/base/deployment.yaml

@@ -46,7 +46,7 @@ spec:
                   name: litellm-secrets
                   key: LITELLM_MASTER_KEY
             - name: NXTGAUGE_USERS_URL
-              value: "http://nxtgauge-rust-users:9101"
+              value: "http://nxtgauge-rust-users.nxtgauge.svc.cluster.local:9101"
             - name: AI_SERVICE_KEY
               valueFrom:
                 secretKeyRef: