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>

Makefile

@@ -1,4 +1,4 @@
-.PHONY: help build test test-unit test-integration test-endpoints test-publish test-all lint lint-fix validate validate-schemas validate-examples check dev-local dev-compose clean publisher
+.PHONY: help build test test-unit test-integration test-endpoints test-publish test-all lint lint-fix validate validate-schemas validate-examples check dev-compose clean publisher
 
 # Default target
 help: ## Show this help message
@@ -15,10 +15,17 @@ publisher: ## Build the publisher tool with version info
 	go build -ldflags="-X main.Version=dev-$(shell git rev-parse --short HEAD) -X main.GitCommit=$(shell git rev-parse HEAD) -X main.BuildTime=$(shell date -u +%Y-%m-%dT%H:%M:%SZ)" -o bin/mcp-publisher ./cmd/publisher
 
 # Test targets
-test-unit: ## Run unit tests with coverage
-	go test -v -race -coverprofile=coverage.out -covermode=atomic -coverpkg=./internal/... ./internal/...
+test-unit: ## Run unit tests with coverage (requires PostgreSQL)
+	@echo "Starting PostgreSQL for unit tests..."
+	@docker compose up -d postgres
+	@echo "Waiting for PostgreSQL to be ready..."
+	@sleep 3
+	@echo "Running unit tests..."
+	go test -v -race -coverprofile=coverage.out -covermode=atomic ./internal/...
 	go tool cover -html=coverage.out -o coverage.html
 	@echo "Coverage report generated: coverage.html"
+	@echo "Stopping PostgreSQL..."
+	@docker compose down postgres
 
 test: ## Run unit tests (use 'make test-all' to run all tests)
 	@echo "⚠️  Running unit tests only. Use 'make test-all' to run both unit and integration tests."
@@ -59,9 +66,6 @@ check: lint validate test-all ## Run all checks (lint, validate, unit tests)
 dev-compose: ## Start development environment with Docker Compose (builds image automatically)
 	docker compose up --build
 
-dev-local: ## Run registry locally
-	go run ./cmd/registry
-
 # Cleanup
 clean: ## Clean build artifacts and coverage files
 	rm -rf bin