feat!: replace postgresql-ha with CloudNativePG

The bundled `postgresql-ha` sub-chart was the only HA-ready database option, but it pinned us to a Bitnami chart
whose images are no longer freely distributed, and it bundled repmgr and pgpool into the release. Operating a
Postgres cluster through a Helm release also means the chart has to reimplement failover, backup and upgrade
orchestration that a dedicated operator already solves.

CloudNativePG is the de-facto standard operator for Postgres on Kubernetes and handles instance management,
streaming replication, switchover and rolling upgrades itself. The operator is deliberately *not* part of this
chart: cluster-scoped CRDs and controllers do not belong into an application release and would conflict with an
operator that is already installed. The chart only renders the namespaced `Cluster` resource, so the operator can
be managed independently.

Because the operator cannot be assumed to be present, `cloudnativePG.enabled` defaults to `false`. Existing users
of `postgresql` or an external database are therefore unaffected.

Credentials follow the chart-wide `secrets.*` convention. Only the password is read from a Secret, because
CloudNativePG requires `bootstrap.initdb.database` and `bootstrap.initdb.owner` as literals in the `Cluster` spec
and cannot resolve them from a Secret. The password is injected into the `init-app-ini` container as
`GITEA__database__PASSWD` instead of being written into the generated `app.ini`.

BREAKING CHANGE: The `postgresql-ha` sub-chart and all `postgresql-ha.*` values have been removed. There is no
in-place migration path between the two deployments: dump the database of the old `postgresql-ha` release with
`pg_dump`, remove the `postgresql-ha` values, deploy the chart with `cloudnativePG.enabled=true` and restore the
dump. Setting `postgresql-ha` now fails the release with a pointer to the changelog.

Co-authored-by: Copilot <copilot@github.com>

docs/ha-setup.md

@@ -28,8 +28,8 @@ In addition, the following components are required for full HA-readiness:
 - A HA-ready cache (`valkey-cluster`)
 - A HA-ready DB
 
-`postgres.enabled`, which default to `true`, must be set to `false` for a HA setup.
-The default `postgres` chart dependency is not HA-ready (there's a dedicated `postgres-ha` chart).
+`postgresql.enabled`, which defaults to `false`, must stay disabled for a HA setup.
+The `postgresql` chart dependency is not HA-ready, use the `cloudnativePG` option instead.
 
 The following sections discuss each of the components in more detail.
 Note that for each component discussed, the shown configurations only provides a (working) starting point, not necessarily the most optimal setup.
@@ -153,15 +153,33 @@ persistence:
 
 ## Database
 
-If you do not have an HA-ready DB, using a managed database service in the cloud might be the easiest and most robust solution.
-Remember: disable the built-in `postgres` dependency and configure the database connection manually via `gitea.config.database`:
+By enabling `cloudnativePG`, the chart renders a [CloudNativePG](https://cloudnative-pg.io/) `Cluster`, which provides a HA-ready PostgreSQL via streaming replication.
+The CloudNativePG operator itself is not part of this chart and has to be [installed cluster-wide](https://cloudnative-pg.io/documentation/current/installation_upgrade/) beforehand.
+For that reason the option is disabled by default.
 
 ```yml
+cloudnativePG:
+  enabled: true
+  instances: 3
+  storage:
+    size: 10Gi
+    storageClass: <storage class>
+  walStorage:
+    enabled: true
+    size: 2Gi
+    storageClass: <storage class>
+```
+
+If you do not want to run the operator, using a managed database service in the cloud might be the easiest and most robust solution.
+Remember: keep `cloudnativePG` and `postgresql` disabled and configure the database connection manually via `gitea.config.database`:
+
+```yml
+cloudnativePG:
+  enabled: false
+postgresql:
+  enabled: false
+
 gitea:
-  database:
-    builtIn:
-      postgresql:
-        enabled: false
   config:
     database:
       DB_TYPE: postgres