feat(images): cache lifetime in config

internal/pkg/config/default.go

@@ -1,10 +1,13 @@
 package config
 
+import "time"
+
 func Default() Cfg {
 	return Cfg{
 		Image: &ImageConfig{
-			Concurency: 10,
-			Quality:    80,
+			Concurency:    10,
+			Quality:       80,
+			CacheLifetime: time.Hour * 24 * 31,
 		},
 		Tracking: &UmamiConfig{
 			ScriptSrc: "https://umami.schreifuchs.ch/script.js",