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>

unittests/helm/config/database.yaml

@@ -0,0 +1,59 @@
+suite: Test config database
+release:
+  name: gitea-unittests
+  namespace: testing
+tests:
+  - it: "Connect to the read-write Service of the CloudNativePG Cluster"
+    template: templates/secret_inlineConfig.yaml
+    set:
+      cloudnativePG:
+        enabled: true
+    asserts:
+      - documentIndex: 0
+        matchRegex:
+          path: data.database
+          decodeBase64: true
+          pattern: HOST=gitea-unittests-postgresql-rw.testing.svc.cluster.local:5432
+
+  - it: "Connect to the read-write Service of the CloudNativePG Cluster with a custom cluster domain"
+    template: templates/secret_inlineConfig.yaml
+    set:
+      clusterDomain: my-special-cluster.local
+      cloudnativePG:
+        enabled: true
+    asserts:
+      - documentIndex: 0
+        matchRegex:
+          path: data.database
+          decodeBase64: true
+          pattern: HOST=gitea-unittests-postgresql-rw.testing.svc.my-special-cluster.local:5432
+
+  - it: "Connect to the configured CloudNativePG database"
+    template: templates/secret_inlineConfig.yaml
+    set:
+      cloudnativePG:
+        enabled: true
+        credentials:
+          database: gitea-database
+          username: gitea-username
+    asserts:
+      - documentIndex: 0
+        equal:
+          path: data.database
+          decodeBase64: true
+          value: |-
+            DB_TYPE=postgres
+            HOST=gitea-unittests-postgresql-rw.testing.svc.cluster.local:5432
+            NAME=gitea-database
+            USER=gitea-username
+
+  - it: "Fail when postgresql and cloudnativePG are enabled at the same time"
+    template: templates/secret_config.yaml
+    set:
+      postgresql:
+        enabled: true
+      cloudnativePG:
+        enabled: true
+    asserts:
+      - failedTemplate:
+          errorMessage: "Only one of postgresql or cloudnativePG can be enabled at the same time."