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/cnpg_pooler.yaml

@@ -31,3 +31,36 @@ spec:
               topologyKey: kubernetes.io/hostname
       containers: []
   type: rw
+---
+apiVersion: postgresql.cnpg.io/v1
+kind: Pooler
+metadata:
+  name: puppet-postgres-pooler-ro
+  namespace: puppet
+spec:
+  cluster:
+    name: puppet-postgres
+  instances: 2
+  pgbouncer:
+    parameters:
+      default_pool_size: "100"
+      max_client_conn: "400"
+    paused: false
+    poolMode: session
+  template:
+    metadata:
+      labels:
+        app: pooler-ro
+    spec:
+      affinity:
+        podAntiAffinity:
+          requiredDuringSchedulingIgnoredDuringExecution:
+            - labelSelector:
+                matchExpressions:
+                  - key: app
+                    operator: In
+                    values:
+                      - pooler-ro
+              topologyKey: kubernetes.io/hostname
+      containers: []
+  type: ro