bskyweb: use avatar thumbnail for post og:image fallback (#9923)

bskyweb/cmd/bskyweb/filters.go

@@ -2,12 +2,14 @@ package main
 
 import (
 	"net/url"
+	"strings"
 
 	"github.com/flosch/pongo2/v6"
 )
 
 func init() {
 	pongo2.RegisterFilter("canonicalize_url", filterCanonicalizeURL)
+	pongo2.RegisterFilter("avatar_thumbnail", filterAvatarThumbnail)
 }
 
 func filterCanonicalizeURL(in *pongo2.Value, param *pongo2.Value) (*pongo2.Value, *pongo2.Error) {
@@ -26,3 +28,8 @@ func filterCanonicalizeURL(in *pongo2.Value, param *pongo2.Value) (*pongo2.Value
 	// Return the cleaned URL
 	return pongo2.AsValue(parsedURL.String()), nil
 }
+
+func filterAvatarThumbnail(in *pongo2.Value, param *pongo2.Value) (*pongo2.Value, *pongo2.Error) {
+	urlStr := in.String()
+	return pongo2.AsValue(strings.Replace(urlStr, "/img/avatar/plain/", "/img/avatar_thumbnail/plain/", 1)), nil
+}