fix(tool-config): merge partial updates instead of replacing, mask bot_app_password

backend/src/services/toolConfig.service.ts

@@ -14,6 +14,8 @@ export interface VikunjaConfig {
 
 export interface NextcloudConfig {
   url: string;
+  bot_username?: string;
+  bot_app_password?: string;
 }
 
 interface CacheEntry<T> {
@@ -72,6 +74,8 @@ async function getNextcloudConfig(): Promise<NextcloudConfig> {
   const db = await getDbConfig('tool_config_nextcloud');
   const config: NextcloudConfig = {
     url: db.url || environment.nextcloudUrl,
+    bot_username: db.bot_username || undefined,
+    bot_app_password: db.bot_app_password || undefined,
   };
 
   nextcloudCache = { data: config, expiresAt: Date.now() + CACHE_TTL_MS };