test: cover chat moderation and the embed endpoints

- CanModerateMessage: group conversations only, chat members only
- parseEmbedRefs, embedKey, escapeLike
- /chat/embeds and /chat/embeds/resolve: 401, 422 before the
  database, 503 without one

classes/chat_context_test.go

@@ -96,6 +96,35 @@ func TestChatContext(t *testing.T) {
 		t.Error("CanActFor is not self-only")
 	}
 
+	// Moderators delete other people's messages in group conversations
+	// (# ogólny) only, and only while they are chat-group members too
+	// (D-10, D-24).
+	for _, mc := range []struct {
+		name     string
+		userID   uint
+		convType string
+		want     bool
+	}{
+		{"moderator in group", 7, "group", true},
+		{"moderator in dm", 7, "dm", false},
+		{"moderator, unknown type", 7, "", false},
+		{"non-moderator in group", 8, "group", false},
+		{"moderator outside the chat group", 9, "group", false},
+		{"no user", 0, "group", false},
+	} {
+		got, err := c.CanModerateMessage(ctx, mc.userID, mc.convType)
+		if err != nil || got != mc.want {
+			t.Errorf("%s: CanModerateMessage = %v, %v, want %v", mc.name, got, err, mc.want)
+		}
+	}
+	if _, err := c.CanModerateMessage(ctx, 13, "group"); err == nil {
+		t.Error("CanModerateMessage with a failing check returned no error")
+	}
+	noMods := classes.NewChatContext(&gorm.DB{}, "studenci", "").WithMembership(members)
+	if ok, err := noMods.CanModerateMessage(ctx, 7, "group"); ok || err != nil {
+		t.Errorf("CanModerateMessage without a moderator group = %v, %v, want false", ok, err)
+	}
+
 	load := classes.ChatContextLoader(func() string { return "studenci" }, func() string { return "moderatorzy" })
 	if got, err := load(ctx, &gorm.DB{}, 2); got != nil || err != nil {
 		t.Errorf("loader(2) = %v, %v, want nil, nil", got, err)