feat: store async generated images as WebP

.env.example

@@ -1,5 +1,8 @@
 ADMIN_SECRET=replace-with-a-long-random-secret
 DB_PATH=./data/proxy.db
+OUTPUT_DIR=./data/generated
+OUTPUT_WEBP_QUALITY=90
+OUTPUT_RETENTION_SECONDS=86400
 MAX_UPLOAD_BYTES=52428800
 MAX_IMAGES=8
 MAX_IMAGE_PIXELS=25000000