fix(cabana): answer an empty or malformed avatar upload body with 422

- POST /account/avatar with no multipart part (NextPart io.EOF) or a broken
  boundary fell through to the generic 500; it is now the invalid-body 422
- a body over the cap still answers 413 payload_too_large

modules/cabana/account.go

@@ -587,7 +587,7 @@ func (s *service) avatarUpload(w http.ResponseWriter, r *http.Request) {
 	}
 	part, err := mr.NextPart()
 	if err != nil {
-		s.writeAvatarError(w, r, body, err)
+		s.writeAvatarError(w, r, body, multipartFailure(body, err))
 		return
 	}
 	if part.FormName() != "file_data" || strings.TrimSpace(part.FileName()) == "" {
@@ -614,7 +614,7 @@ func (s *service) avatarUpload(w http.ResponseWriter, r *http.Request) {
 			if err == nil {
 				return invalidBody()
 			}
-			return err
+			return multipartFailure(body, err)
 		}
 		q := tx.Session(&gorm.Session{NewDB: true, Context: ctx})
 		if err := q.Model(&attach.File{}).Where("id = ?", f.ID).Updates(map[string]any{
@@ -680,6 +680,17 @@ func (s *service) avatarRemove(w http.ResponseWriter, r *http.Request) {
 	WriteData(w, http.StatusOK, s.profileFor(r.Context(), db, saved), map[string]any{})
 }
 
+// multipartFailure is the error of a multipart read: a body past the cap
+// stays a *http.MaxBytesError (413), and an empty or malformed body (no
+// part at all, a broken boundary) is the invalid-body 422, never a 500.
+func multipartFailure(body *bodyReader, err error) error {
+	var tooBig *http.MaxBytesError
+	if errors.As(err, &tooBig) || (body != nil && errors.As(body.err, &tooBig)) {
+		return err
+	}
+	return invalidBody()
+}
+
 // writeAvatarError maps an avatar upload failure as writeFileError does,
 // with the field name avatar: a body past the cap is 413, an attach.Store
 // refusal a 422 on avatar and a malformed multipart body a 422 on body.