Migrate dev server from MongoDB to SQLite

- Replace mongooseAdapter with sqliteAdapter in payload config
- Update database configuration to use file:./dev.db
- Remove MongoDB memory database helper and references
- Simplify start script by removing verbose logging and MongoDB messaging
- Fix email processing with immediate send support and proper queue handling
- Restructure app with route groups for frontend/admin separation
- Add dashboard and test pages for email management
- Update API routes for improved email processing and testing

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

dev/app/page.tsx

@@ -0,0 +1,11 @@
+import { Metadata } from 'next'
+import {redirect} from "next/navigation.js"
+
+export const metadata: Metadata = {
+  title: 'PayloadCMS Mailing Plugin - Development',
+  description: 'Development environment for PayloadCMS Mailing Plugin',
+}
+
+export default function HomePage() {
+  redirect('/dashboard')
+}