This commit refactors the API server startup logic in `cmd/api/main.go` to simplify the application's architecture.
Key changes:
- Consolidates the three separate HTTP servers (GraphQL API, GraphQL Playground, and Prometheus metrics) into a single `http.Server` instance.
- Uses a single `http.ServeMux` to route requests to the appropriate handlers on distinct paths (`/query`, `/playground`, `/metrics`).
- Removes the now-redundant `PlaygroundPort` from the application's configuration.
This change simplifies the server startup and shutdown logic, reduces resource usage, and makes the application's entry point cleaner and easier to maintain.
