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/README.md

@@ -11,7 +11,7 @@ Pre-requisites:
 - Access to a Kubernetes cluster via kubeconfig. You can run a cluster locally with [minikube](https://minikube.sigs.k8s.io/docs/start/).
 
 1. Ensure your kubeconfig is configured at the cluster you want to use. For minikube, run `minikube start && minikube tunnel`.
-2. Run `make local-up` to deploy the stack. Run this again if the first attempt fails.
+2. Run `make local-up` to deploy the stack.
 3. Access the repository via the ingress load balancer. You can find its external IP with `kubectl get svc ingress-nginx-controller -n ingress-nginx`. Then run `curl -H "Host: local.registry.modelcontextprotocol.io" -k https://<EXTERNAL-IP>/v0/ping` to check that the service is up.
 
 #### To change config
@@ -103,7 +103,7 @@ Pre-requisites:
     │   ├── cert_manager.go    # SSL certificate management
     │   ├── deploy.go          # Deployment orchestration
     │   ├── ingress.go         # Ingress controller setup
-    │   ├── mongodb.go         # MongoDB deployment
+    │   ├── postgres.go        # PostgreSQL database deployment
     │   └── registry.go        # MCP Registry deployment
     └── providers/       # Kubernetes cluster providers
         ├── types.go           # Provider interface definitions
@@ -122,7 +122,7 @@ Pre-requisites:
 5. `k8s.DeployAll()` orchestrates complete deployment:
    - Certificate manager for SSL/TLS
    - Ingress controller for external access
-   - MongoDB for data persistence
+   - Database for data persistence
    - MCP Registry application
 
 ## Configuration
@@ -152,5 +152,5 @@ kubectl get svc -n ingress-nginx
 
 ```bash
 kubectl logs -l app=mcp-registry
-kubectl logs -l app=mongodb
+kubectl logs -l app=postgres
 ```