Refuse a signed PORT such as +9090

strconv.Atoi accepts a leading sign, so PORT=+9090 passed the check
while the image's health check put "+9090" into its URL and failed.
PORT is now parsed with strconv.ParseUint, which accepts only plain
digits.

Model: opus-5-5

internal/config/config.go

@@ -122,12 +122,13 @@ func getPort() (int, error) {
 		return defaultPort, nil
 	}
 
-	port, err := strconv.Atoi(value)
+	// ParseUint, unlike Atoi, refuses a sign: the health check URL cannot use "+9090"
+	port, err := strconv.ParseUint(value, 10, 0)
 	if err != nil || port < 1 || port > maxPort {
 		return 0, fmt.Errorf("PORT must be a whole number from 1 to %d, got %q", maxPort, value)
 	}
 
-	return port, nil
+	return int(port), nil
 }
 
 // EnsureDirectories creates all necessary directories if they don't exist