refactor: centralize avatar URL generation and backfill existing users

internal/users/avatar.go

@@ -0,0 +1,11 @@
+package users
+
+import (
+	"net/url"
+	"strings"
+)
+
+func DefaultAvatarURL(username string) string {
+	seed := url.QueryEscape(strings.TrimSpace(username))
+	return "https://api.dicebear.com/9.x/dylan/svg?seed=" + seed
+}