Default max_concurrent_processing to half the CPUs pixa sees (closes #242)

The default max_concurrent_processing was one image per CPU pixa sees,
so in a container with no CPU limit on a 48-CPU host pixa processed 48
images at once. It is now half the CPUs, rounded down and at least 1,
defined once in internal/config; PIXA_MAX_CONCURRENT_PROCESSING stays
the override, and a value that is not a whole number of at least 1
stops startup. "Running under upaas" gives the worst-case memory, 4.2
GiB per image (the largest peak measured over every source format and
kind at 8192x8192) plus the upstream buffers, and the temporary file
libvips keeps under /tmp. The two existing tests expect the new
default, as the owner allowed.

Model: opus-5-5

internal/config/config.go

@@ -37,6 +37,14 @@ const (
 	DefaultImageRequestsPerMinute     = 600
 )
 
+// DefaultMaxConcurrentProcessing returns the most images processed at once
+// when max_concurrent_processing is omitted: half the CPUs Go uses
+// (runtime.GOMAXPROCS(0), which follows a container's CPU limit), rounded
+// down, and at least 1.
+func DefaultMaxConcurrentProcessing() int {
+	return max(runtime.GOMAXPROCS(0)/2, 1) //nolint:mnd // half the CPUs
+}
+
 // Configuration key names.
 const (
 	keyDebug                      = "debug"
@@ -297,10 +305,8 @@ func newFromSmartConfig(sc *smartconfig.Config) (*Config, error) {
 			keyUpstreamConnectionsPerHost, DefaultUpstreamConnectionsPerHost),
 		UpstreamConnections: loader.intVal(
 			keyUpstreamConnections, DefaultUpstreamConnections),
-		// Decoding and encoding are CPU-bound, so the default is one image
-		// per CPU Go uses, which follows a container's CPU limit.
 		MaxConcurrentProcessing: loader.intVal(
-			keyMaxConcurrentProcessing, runtime.GOMAXPROCS(0)),
+			keyMaxConcurrentProcessing, DefaultMaxConcurrentProcessing()),
 		UpstreamFetchTimeout: loader.durationVal(
 			keyUpstreamFetchTimeout, DefaultUpstreamFetchTimeout),
 		UpstreamMaxResponseSize: loader.int64Val(