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.

tests/integration/docker-compose.integration-test.yml

@@ -1,13 +1,13 @@
 services:
   registry:
     environment:
-      - MCP_REGISTRY_SEED_IMPORT=false
+      - MCP_REGISTRY_SEED_FROM=
     healthcheck:
       test: ["CMD", "wget", "-qO-", "http://localhost:8080/v0/servers"]
       interval: 1s
       retries: 60
       timeout: 1s
-  mongodb:
+  postgres:
     volumes: !reset []
     tmpfs:
-      - /data/db:size=50m
+      - /var/lib/postgresql/data:size=50m