feat(config): make resolver support null value to delete a

key.

config/settings.go

@@ -121,9 +121,15 @@ func UpdateSettings(settingsType string, patch map[string]interface{}) error {
 
 // deepMergeSettings recursively overlays src onto dst. Object values merge
 // key-by-key so partial updates keep sibling keys untouched; arrays and scalars
-// always replace the destination value.
+// always replace the destination value. A JSON null in the payload removes that
+// key from dst, giving clients a way to delete entries (members, nodes, map
+// rows) through /api/settings/set.
 func deepMergeSettings(dst, src map[string]interface{}) {
 	for key, srcVal := range src {
+		if srcVal == nil {
+			delete(dst, key)
+			continue
+		}
 		srcObj, srcIsObj := srcVal.(map[string]interface{})
 		if srcIsObj {
 			if dstObj, ok := dst[key].(map[string]interface{}); ok {

config/settings_test.go

@@ -176,3 +176,46 @@ func TestGetSettingsBotNodeMissingFile(t *testing.T) {
 		t.Errorf("expected empty object for missing bot_node_config, got %s", data)
 	}
 }
+
+func TestUpdateSettingsRemovesKeysWithNull(t *testing.T) {
+	writeTempConfig(t, &global.ConfigPath.BotUserConfig, `{
+    "qq(napcat)": {
+        "admins": {
+            "3526453517": { "event_status_notify": true },
+            "740724778": { "event_status_notify": false }
+        },
+        "trustedGroups": {
+            "999": { "event_bot_started": true }
+        }
+    },
+    "telegram": {
+        "admins": {}
+    }
+}`)
+
+	// null removes a nested member and keeps its siblings; an empty section stays.
+	patch := map[string]interface{}{
+		"qq(napcat)": map[string]interface{}{
+			"admins": map[string]interface{}{
+				"3526453517": nil,
+			},
+		},
+	}
+	if err := UpdateSettings(SettingBotUserConfig, patch); err != nil {
+		t.Fatalf("UpdateSettings: %v", err)
+	}
+
+	data, err := GetSettings(SettingBotUserConfig)
+	if err != nil {
+		t.Fatalf("GetSettings: %v", err)
+	}
+	got := string(data)
+	if strings.Contains(got, "3526453517") {
+		t.Errorf("deleted member still present:\n%s", got)
+	}
+	for _, want := range []string{"740724778", `"trustedGroups"`, `"telegram"`} {
+		if !strings.Contains(got, want) {
+			t.Errorf("unrelated content missing %q:\n%s", want, got)
+		}
+	}
+}

database/user.go

@@ -108,7 +108,7 @@ func RegisterFirstUser(username, password, level string) (int64, error) {
 		return 0, fmt.Errorf("failed to check existing users: %w", err)
 	}
 	if count > 0 {
-		return 0, errors.New("registration closed: users already exist")
+		return 0, ErrUsersExist
 	}
 
 	result, err := tx.Exec(