docs: update Node.js version requirements to 20+ across all documentation

Co-authored-by: hotlong <50353452+hotlong@users.noreply.github.com>

Author: Copilot

content/docs/admin/setup.mdx

@@ -11,7 +11,7 @@ This guide walks system administrators through the initial setup and ongoing con
 
 Before you begin, ensure you have:
 
-- **Node.js** 18+ installed
+- **Node.js** 20+ installed
 - **pnpm** package manager
 - Access to a supported database (PostgreSQL recommended)
 - Administrator credentials for your HotCRM instance

content/docs/architecture/overview.mdx

@@ -312,7 +312,7 @@ Load Balancer
 
 ### Backend
 
-- **Runtime**: Node.js 18+
+- **Runtime**: Node.js 20+
 - **Language**: TypeScript 5+
 - **Framework**: Express.js
 - **Package Manager**: pnpm 8+