feat(lagoon): WebP variants of attachment thumbnails

- attach.VariantKey: a thumbnail's copy in another format next to it
- attach/webp (separate package, so only binaries that import it link
  the encoder): Encode at quality 80 and Variant, which writes the WebP
  copy of a thumbnail once and leaves no file on a failed encode
- github.com/gen2brain/webp v0.6.4, decided in
  .planning/notes/2026-10-28-webp-gen2brain.md
- lagoon README and docs/database/attachments.md

modules/lagoon/attach/webp/export_test.go

@@ -0,0 +1,13 @@
+package webp
+
+import (
+	"image"
+	"io"
+)
+
+// SetEncoderForTest replaces the encoder Variant uses and returns the
+// restore function (tests only).
+func SetEncoderForTest(fn func(io.Writer, image.Image) error) func() {
+	encodeImage = fn
+	return func() { encodeImage = Encode }
+}