feat: make CheckPorts concurrent and add port validation

- CheckPorts now runs all port checks concurrently using errgroup
- Added port number validation (1-65535) with ErrInvalidPort sentinel error
- Updated PortChecker interface to use *PortResult return type
- Added tests for invalid port numbers (0, negative, >65535)
- All checks pass (make check clean)

internal/watcher/interfaces.go

@@ -4,6 +4,7 @@ package watcher
 import (
 	"context"
 
+	"sneak.berlin/go/dnswatcher/internal/portcheck"
 	"sneak.berlin/go/dnswatcher/internal/tlscheck"
 )
 
@@ -36,7 +37,7 @@ type PortChecker interface {
 		ctx context.Context,
 		address string,
 		port int,
-	) (bool, error)
+	) (*portcheck.PortResult, error)
 }
 
 // TLSChecker inspects TLS certificates.