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>

README.md

@@ -39,24 +39,9 @@ Often (but not always) ideas flow through this pipeline:
 make dev-compose
 ```
 
-This starts the registry at [`localhost:8080`](http://localhost:8080) with PostgreSQL and seed data. It can be configured with environment variables in [docker-compose.yml](./docker-compose.yml) - see [.env.example](./.env.example) for a reference.
+This starts the registry at [`localhost:8080`](http://localhost:8080) with PostgreSQL and seed data. The database uses ephemeral storage and is reset each time you restart the containers, ensuring a clean state for development and testing.
 
-<details>
-<summary>Alternative: Local setup without Docker</summary>
-
-**Prerequisites:**
-- PostgreSQL running locally
-- Go 1.24.x installed
-
-```bash
-# Build and run locally
-make build
-make dev-local
-```
-
-The service runs on [`localhost:8080`](http://localhost:8080) by default. This can be configured with environment variables in `.env` - see [.env.example](./.env.example) for a reference.
-
-</details>
+The setup can be configured with environment variables in [docker-compose.yml](./docker-compose.yml) - see [.env.example](./.env.example) for a reference.
 
 <details>
 <summary>Alternative: Running a pre-built Docker image</summary>
@@ -125,7 +110,7 @@ For Claude and other AI tools: Always prefer make targets over custom commands w
 │   ├── api/                 # HTTP handlers and routing
 │   ├── auth/                # Authentication (GitHub OAuth, JWT, namespace blocking)
 │   ├── config/              # Configuration management
-│   ├── database/            # Data persistence (PostgreSQL, in-memory)
+│   ├── database/            # Data persistence (PostgreSQL)
 │   ├── service/             # Business logic
 │   ├── telemetry/           # Metrics and monitoring
 │   └── validators/          # Input validation