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/deploy.go

@@ -16,19 +16,19 @@ func DeployAll(ctx *pulumi.Context, cluster *providers.ProviderInfo, environment
 	}
 
 	// Setup ingress controller
-	err = SetupIngressController(ctx, cluster, environment)
+	ingressNginx, err := SetupIngressController(ctx, cluster, environment)
 	if err != nil {
 		return nil, err
 	}
 
-	// Deploy MongoDB
-	err = DeployMongoDB(ctx, cluster, environment)
+	// Deploy PostgreSQL databases
+	pgCluster, err := DeployPostgresDatabases(ctx, cluster, environment)
 	if err != nil {
 		return nil, err
 	}
 
 	// Deploy MCP Registry
-	service, err = DeployMCPRegistry(ctx, cluster, environment)
+	service, err = DeployMCPRegistry(ctx, cluster, environment, ingressNginx, pgCluster)
 	if err != nil {
 		return nil, err
 	}