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_cluster.yaml

@@ -19,6 +19,24 @@ spec:
       postInitApplicationSQL:
         - CREATE EXTENSION IF NOT EXISTS pg_trgm;
         - CREATE EXTENSION IF NOT EXISTS pgcrypto;
+        - GRANT CONNECT ON DATABASE puppetdb TO puppetdb_read;
+        - GRANT USAGE ON SCHEMA public TO puppetdb_read;
+        - GRANT SELECT ON ALL TABLES IN SCHEMA public TO puppetdb_read;
+        - ALTER DEFAULT PRIVILEGES IN SCHEMA public GRANT SELECT ON TABLES TO puppetdb_read;
+  managed:
+    roles:
+      - name: puppetdb_read
+        ensure: present
+        comment: PuppetDB read-only database user
+        login: true
+        superuser: false
+        createdb: false
+        createrole: false
+        inherit: true
+        replication: false
+        connectionLimit: -1
+        passwordSecret:
+          name: postgres-read-credentials
   enablePDB: true
   enableSuperuserAccess: false
   failoverDelay: 0