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

go.sum

@@ -173,6 +173,8 @@ github.com/fsnotify/fsnotify v1.10.1 h1:b0/UzAf9yR5rhf3RPm9gf3ehBPpf0oZKIjtpKrx5
 github.com/fsnotify/fsnotify v1.10.1/go.mod h1:TLheqan6HD6GBK6PrDWyDPBaEV8LspOxvPSjC+bVfgo=
 github.com/gabriel-vasile/mimetype v1.4.15 h1:05iP/CYtZ/w455R/KZM6rZ5ieAdh99UPtd+d3YzLmaI=
 github.com/gabriel-vasile/mimetype v1.4.15/go.mod h1:azpTcoLcDZRNgFou5j+APrqQx9HqVPWa6ijYQIIVswQ=
+github.com/gen2brain/webp v0.6.4 h1:SUDdmxADOAiPQ+5ylNmuHhuYf2dOi0KgKZHL5vpVCNU=
+github.com/gen2brain/webp v0.6.4/go.mod h1:iGWMaCSw7t3I/Cv9llzEKmpnR36S8lS8VL/ZVjxU0JE=
 github.com/go-gormigrate/gormigrate/v2 v2.1.7 h1:PdT4jVPbRb4R+0Ey2R0yJOdctVf4Whiq1Qi4necaZdg=
 github.com/go-gormigrate/gormigrate/v2 v2.1.7/go.mod h1:3ouXglTuPrKF5+7cQyVGfvAXTU4vLMaYh9+EPl03uog=
 github.com/go-jose/go-jose/v4 v4.1.4 h1:moDMcTHmvE6Groj34emNPLs/qtYXRVcd6S7NHbHz3kA=