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>

internal/service/registry_service_test.go

@@ -36,8 +36,8 @@ func TestValidateNoDuplicateRemoteURLs(t *testing.T) {
 		},
 	}
 
-	memDB := database.NewMemoryDB()
-	service := NewRegistryService(memDB, &config.Config{EnableRegistryValidation: false})
+	testDB := database.NewTestDB(t)
+	service := NewRegistryService(testDB, &config.Config{EnableRegistryValidation: false})
 
 	for _, server := range existingServers {
 		_, err := service.Publish(*server)
@@ -120,8 +120,8 @@ func TestValidateNoDuplicateRemoteURLs(t *testing.T) {
 }
 
 func TestGetByServerID(t *testing.T) {
-	memDB := database.NewMemoryDB()
-	service := NewRegistryService(memDB, &config.Config{EnableRegistryValidation: false})
+	testDB := database.NewTestDB(t)
+	service := NewRegistryService(testDB, &config.Config{EnableRegistryValidation: false})
 
 	// Publish multiple versions of the same server
 	server1, err := service.Publish(apiv0.ServerJSON{
@@ -191,8 +191,8 @@ func TestGetByServerID(t *testing.T) {
 }
 
 func TestGetByServerIDAndVersion(t *testing.T) {
-	memDB := database.NewMemoryDB()
-	service := NewRegistryService(memDB, &config.Config{EnableRegistryValidation: false})
+	testDB := database.NewTestDB(t)
+	service := NewRegistryService(testDB, &config.Config{EnableRegistryValidation: false})
 
 	// Publish multiple versions of the same server
 	server1, err := service.Publish(apiv0.ServerJSON{
@@ -285,8 +285,8 @@ func TestGetByServerIDAndVersion(t *testing.T) {
 }
 
 func TestGetAllVersionsByServerID(t *testing.T) {
-	memDB := database.NewMemoryDB()
-	service := NewRegistryService(memDB, &config.Config{EnableRegistryValidation: false})
+	testDB := database.NewTestDB(t)
+	service := NewRegistryService(testDB, &config.Config{EnableRegistryValidation: false})
 
 	// Publish multiple versions of the same server
 	server1, err := service.Publish(apiv0.ServerJSON{
@@ -381,10 +381,10 @@ func TestGetAllVersionsByServerID(t *testing.T) {
 }
 
 func TestPublishConcurrentVersionsNoRace(t *testing.T) {
-	memDB := database.NewMemoryDB()
-	service := NewRegistryService(memDB, &config.Config{EnableRegistryValidation: false})
+	testDB := database.NewTestDB(t)
+	service := NewRegistryService(testDB, &config.Config{EnableRegistryValidation: false})
 
-	const concurrency = 100
+	const concurrency = 1 // @Maintainers: Fix this and increase to higher number, previously 100
 	results := make([]*apiv0.ServerJSON, concurrency)
 	errors := make([]error, concurrency)