Say half the CPUs plainly in DefaultMaxConcurrentProcessing

The constant cpusPerImage gave a reason nothing else states, that each
image uses two CPUs. The function now divides by 2, with the nolint
comment the repo uses for such numbers.

Model: opus-5-5

internal/config/config.go

@@ -42,9 +42,7 @@ const (
 // (runtime.GOMAXPROCS(0), which follows a container's CPU limit), rounded
 // down, and at least 1.
 func DefaultMaxConcurrentProcessing() int {
-	const cpusPerImage = 2
-
-	return max(runtime.GOMAXPROCS(0)/cpusPerImage, 1)
+	return max(runtime.GOMAXPROCS(0)/2, 1) //nolint:mnd // half the CPUs
 }
 
 // Configuration key names.