Remove the in-memory database (#537)

<!-- Provide a brief summary of your changes -->

## Motivation and Context
<!-- Why is this change needed? What problem does it solve? -->
The follow PR removes the in-memory database option from the registry.

We had a chat about this with @domdomegg and although it is supposed to
be convenient, it isn't that useful in reality. We want to test and
develop the registry in a setup that is closer to the environment it is
supposed to be run. One of the main reasons are:
* We don't use it anywhere else besides unit testing and potentially
local development. Instead we should use postgresql for reliable results
(in-memory was actually giving false positive results)
* We have to keep maintaining it and make sure it mimics the behaviour
we have for postgresql which is not yielding a lot of value

## How Has This Been Tested?
<!-- Have you tested this in a real application? Which scenarios were
tested? -->
Yes, I've updated the test unit make target to run in the postgresql
environment and now we can see the race condition issue happening.

## Breaking Changes
<!-- Will users need to update their code or configurations? -->
No

## 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)
- [ ] 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. -->
- [ ] I have read the [MCP
Documentation](https://modelcontextprotocol.io)
- [ ] My code follows the repository's style guidelines
- [ ] New and existing tests pass locally
- [ ] I have added appropriate error handling
- [ ] I have added or updated documentation as needed

## Additional context
<!-- Add any other context, implementation notes, or design decisions
-->

---------

Signed-off-by: Radoslav Dimitrov <radoslav@stacklok.com>

docker-compose.yml

@@ -36,8 +36,6 @@ services:
       POSTGRES_USER: mcpregistry
       POSTGRES_PASSWORD: mcpregistry
       PGDATA: /var/lib/postgresql/data/pgdata
-    volumes:
-      - './.db:/var/lib/postgresql/data'
     healthcheck:
       test: ["CMD-SHELL", "pg_isready -U mcpregistry -d mcp-registry"]
       interval: 1s