Refactor database implementation to use PostgreSQL instead of MongoDB (#289)

Fixes #228

## Motivation and Context

See #19

## How Has This Been Tested?

- Run locally
- Run in docker compose
- Run infra in minikube
- Unit and integration tests all passing :)

## Breaking Changes

Yes: users will not be able to access any data they used to have in
custom mongodb. We are accepting this breaking change, given the
registry is in experimental development status.

## Types of changes
<!-- What types of changes does your code introduce? Put an `x` in all
the boxes that apply: -->
- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [x] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Documentation update

## Checklist
<!-- Go over all the following points, and put an `x` in all the boxes
that apply. -->
- [x] I have read the [MCP
Documentation](https://modelcontextprotocol.io)
- [x] My code follows the repository's style guidelines
- [x] New and existing tests pass locally
- [x] I have added appropriate error handling
- [x] I have added or updated documentation as needed

## Additional context

I have not implemented connection pooling support for simplicity, and
because cloudnative-pg already provides built-in connection pooling with
pgBouncer so if we want it later we can switch to this endpoint easily.

deploy/pkg/k8s/postgres.go

@@ -0,0 +1,70 @@
+package k8s
+
+import (
+	"github.com/pulumi/pulumi-kubernetes/sdk/v4/go/kubernetes/apiextensions"
+	corev1 "github.com/pulumi/pulumi-kubernetes/sdk/v4/go/kubernetes/core/v1"
+	"github.com/pulumi/pulumi-kubernetes/sdk/v4/go/kubernetes/helm/v3"
+	metav1 "github.com/pulumi/pulumi-kubernetes/sdk/v4/go/kubernetes/meta/v1"
+	"github.com/pulumi/pulumi/sdk/v3/go/pulumi"
+
+	"github.com/modelcontextprotocol/registry/deploy/infra/pkg/providers"
+)
+
+// DeployPostgresDatabases deploys the CloudNative PostgreSQL operator and PostgreSQL cluster
+func DeployPostgresDatabases(ctx *pulumi.Context, cluster *providers.ProviderInfo, environment string) (*apiextensions.CustomResource, error) {
+	// Create cnpg-system namespace
+	cnpgNamespace, err := corev1.NewNamespace(ctx, "cnpg-system", &corev1.NamespaceArgs{
+		Metadata: &metav1.ObjectMetaArgs{
+			Name: pulumi.String("cnpg-system"),
+		},
+	}, pulumi.Provider(cluster.Provider))
+	if err != nil {
+		return nil, err
+	}
+
+	// Install cloudnative-pg Helm chart
+	cloudNativePG, err := helm.NewChart(ctx, "cloudnative-pg", helm.ChartArgs{
+		Chart:   pulumi.String("cloudnative-pg"),
+		Version: pulumi.String("v0.26.0"),
+		FetchArgs: helm.FetchArgs{
+			Repo: pulumi.String("https://cloudnative-pg.github.io/charts"),
+		},
+		Namespace: cnpgNamespace.Metadata.Name().Elem(),
+		Values: pulumi.Map{
+			"webhooks": pulumi.Map{
+				"replicaCount": pulumi.Int(1),
+			},
+		},
+	}, pulumi.Provider(cluster.Provider))
+	if err != nil {
+		return nil, err
+	}
+
+	// Create PostgreSQL cluster with proper timeout handling
+	// Note: This may fail on first run until CloudNativePG operator is fully ready
+	pgCluster, err := apiextensions.NewCustomResource(ctx, "registry-pg", &apiextensions.CustomResourceArgs{
+		ApiVersion: pulumi.String("postgresql.cnpg.io/v1"),
+		Kind:       pulumi.String("Cluster"),
+		Metadata: &metav1.ObjectMetaArgs{
+			Name:      pulumi.String("registry-pg"),
+			Namespace: pulumi.String("default"),
+			Labels: pulumi.StringMap{
+				"app":         pulumi.String("registry-pg"),
+				"environment": pulumi.String(environment),
+			},
+		},
+		OtherFields: map[string]any{
+			"spec": map[string]any{
+				"instances": 1,
+				"storage": map[string]any{
+					"size": "50Gi",
+				},
+			},
+		},
+	}, pulumi.Provider(cluster.Provider), pulumi.DependsOnInputs(cloudNativePG.Ready))
+	if err != nil {
+		return nil, err
+	}
+
+	return pgCluster, nil
+}