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_test.go

@@ -31,7 +31,7 @@ func TestNewReadsPort(t *testing.T) {
 }
 
 func TestNewRefusesInvalidPort(t *testing.T) {
-	for _, value := range []string{"0", "65536", "-1", "http", "80.5"} {
+	for _, value := range []string{"0", "65536", "-1", "+9090", "http", "80.5"} {
 		t.Run(value, func(t *testing.T) {
 			t.Setenv("PORT", value)
 			t.Setenv("XDG_DATA_HOME", "")