feat: add PuppetDB read-only database user and pooler

PuppetDB requires a separate read-only database user for its read pool.
Without it, it refuses to use the write user for read queries and all
/pdb/query/v4 calls fail with a 500.

- Add puppetdb_read role via CNPG managed.roles with password sourced
  from a new postgres-read-credentials Vault secret
- Grant CONNECT, USAGE, SELECT and default privileges to puppetdb_read
  via postInitApplicationSQL (must also be run manually on existing cluster)
- Add puppet-postgres-pooler-ro Pooler (type: ro) routing to replicas
- Add puppetdb-read-database-conf ConfigMap with read-database.conf
  mounted into /etc/puppetlabs/puppetdb/conf.d/ in the PuppetDB deployment
- Wire OPENVOXDB_READ_POSTGRES_* env vars from the new secret

💘 Generated with Crush

Assisted-by: Claude Sonnet 4.6 via Crush <crush@charm.land>

apps/base/puppet/configmap_puppetdb-read-database.yaml

@@ -0,0 +1,18 @@
+---
+apiVersion: v1
+kind: ConfigMap
+metadata:
+  labels:
+    app.kubernetes.io/component: puppetdb
+    app.kubernetes.io/instance: puppetserver
+    app.kubernetes.io/name: puppetserver
+    app.kubernetes.io/version: 8.8.0
+  name: puppetdb-read-database-conf
+  namespace: puppet
+data:
+  read-database.conf: |
+    read-database: {
+      subname: "//"${OPENVOXDB_READ_POSTGRES_HOSTNAME}":"${OPENVOXDB_READ_POSTGRES_PORT}"/"${OPENVOXDB_READ_POSTGRES_DATABASE}
+      username: ${OPENVOXDB_READ_POSTGRES_USER}
+      password: ${OPENVOXDB_READ_POSTGRES_PASSWORD}
+    }