tower 0.61.6 — hide platform cluster from tenant-scoped server lists

handleListServers's existing tenant filter would still admit a
platform cluster (type='platform') if it had been mis-labeled with
a tenant or if a tenant happened to host an instance there. Belt-
and-braces: explicit reject of any cluster with type='platform'
when the request is tenant-scoped (non-super-admin). The platform
control-plane runs Tower itself + platform-tenant template builds
— it is not a deployment target for customer tenants and surfacing
it in their Server picker breaks the bring-your-own-cluster model.

Caught while smoke-testing MigrateDrawer: a fresh tenant's Server
dropdown defaulted to 'Platform server', risking a customer
deploying their tenant data onto the operator's shared infra by
accident.

values.yaml

@@ -9,7 +9,7 @@ backend:
     # so every cluster that runs Tower needs the same imagePullSecret
     # provisioned out-of-band (until cluster-platform-v3 owns it).
     repository: registry.odoosky.cloud/odoosky/docker-mirror/tower
-    tag: "0.61.5"
+    tag: "0.61.6"
     pullPolicy: IfNotPresent
   imagePullSecrets:
     - name: docker-mirror-pull