fix: use CAMELEER_API_URL env var for server-ui container

The nginx template in cameleer3-server-ui uses ${CAMELEER_API_URL} for
the upstream proxy target, not API_URL. The wrong env var name caused
the baked-in default (http://cameleer3-server:8081) to be used, which
doesn't resolve in per-tenant networks where the server is named
cameleer-server-{slug}.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitignore

@@ -25,3 +25,4 @@ Thumbs.db
 # Generated by postinstall from @cameleer/design-system
 ui/public/favicon.svg
 docker/runtime-base/agent.jar
+.gitnexus

src/main/java/net/siegeln/cameleer/saas/provisioning/DockerTenantProvisioner.java

@@ -211,7 +211,7 @@ public class DockerTenantProvisioner implements TenantProvisioner {
 
         List<String> env = List.of(
             "BASE_PATH=" + prefix,
-            "API_URL=http://" + serverName + ":8081"
+            "CAMELEER_API_URL=http://" + serverName + ":8081"
         );
 
         // Primary network = tenant network (can reach server via DNS)