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.

internal/config/config.go

@@ -7,16 +7,16 @@ import (
 type DatabaseType string
 
 const (
-	DatabaseTypeMongoDB DatabaseType = "mongodb"
-	DatabaseTypeMemory  DatabaseType = "memory"
+	DatabaseTypePostgreSQL DatabaseType = "postgresql"
+	DatabaseTypeMemory     DatabaseType = "memory"
 )
 
 // Config holds the application configuration
 // See .env.example for more documentation
 type Config struct {
 	ServerAddress       string       `env:"SERVER_ADDRESS" envDefault:":8080"`
-	DatabaseType        DatabaseType `env:"DATABASE_TYPE" envDefault:"mongodb"`
-	DatabaseURL         string       `env:"DATABASE_URL" envDefault:"mongodb://localhost:27017"`
+	DatabaseType        DatabaseType `env:"DATABASE_TYPE" envDefault:"postgresql"`
+	DatabaseURL         string       `env:"DATABASE_URL" envDefault:"postgres://localhost:5432/mcp-registry?sslmode=disable"`
 	DatabaseName        string       `env:"DATABASE_NAME" envDefault:"mcp-registry"`
 	CollectionName      string       `env:"COLLECTION_NAME" envDefault:"servers_v2"`
 	LogLevel            string       `env:"LOG_LEVEL" envDefault:"info"`