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.

docs/architecture.md

@@ -14,7 +14,7 @@ graph TB
     
     subgraph "MCP Registry"
         API[REST API<br/>Go]
-        DB[(MongoDB or PostgreSQL)]
+        DB[(PostgreSQL)]
         CDN[CDN Cache]
     end
     
@@ -61,7 +61,7 @@ The main application server implemented in Go, providing:
 - GitHub OAuth integration (extensible to other providers)
 - DNS verification system (optional for custom namespaces)
 
-### Database (MongoDB or PostgreSQL)
+### Database (PostgreSQL)
 
 Primary data store for:
 - Versioned server metadata (server.json contents)