Check required ports are reachable

[vidya381]

Check if something is listening on the ports commonly used by detected services:

• Postgres → 5432
• Redis → 6379
• MySQL → 3306
• MongoDB → 27017

Try a TCP dial on localhost for each detected service. If nothing is listening, fail with a hint to start the service.

Add internal/check/port.go and register under each relevant stack condition.