feat: migrate from Next.js Pages Router to App Router with Payload CMS

Complete architectural overhaul migrating from Directus+tRPC to Payload CMS with Next.js App Router. This represents a fundamental shift in how the application is structured and how data is managed.

Major changes:
- Migrated from Pages Router (src/pages/) to App Router (src/app/)
- Replaced Directus CMS with Payload CMS as the content management system
- Removed tRPC in favor of Payload's built-in API routes
- Added PostgreSQL database via Docker Compose for local development
- Implemented separate route groups for frontend and Payload admin
- Updated all API routes to App Router conventions
- Added Payload collections for Projects, Technologies, Links, Media, and Users
- Configured ESLint for new project structure

Infrastructure:
- Added docker-compose.yml for PostgreSQL database
- Updated environment variables for Payload CMS configuration
- Integrated @payloadcms/next for seamless Next.js integration
- Added GraphQL API and playground routes

Dependencies:
- Upgraded React from 18.2.0 to 19.2.0
- Upgraded Next.js to 15.5.6
- Added Payload CMS 3.x packages (@payloadcms/db-postgres, @payloadcms/next, etc.)
- Removed Directus SDK and tRPC packages
- Updated Sharp to 0.34.x
- Migrated to @tanstack/react-query v5

.env.example

@@ -6,7 +6,20 @@
 
 # When adding additional env variables, the schema in /env/schema.mjs should be updated accordingly
 
-# Example:
-# SERVERVAR=foo
-# NEXT_PUBLIC_CLIENTVAR=bar
-DIRECTUS_REVALIDATE_KEY=
+# Payload CMS
+PAYLOAD_SECRET=your-secret-key-here
+DATABASE_URI=postgresql://xevion:xevion_dev_password@localhost:5432/xevion_dev
+PAYLOAD_REVALIDATE_KEY=your-revalidate-key-here
+
+# GitHub API (for cron job)
+GITHUB_API_TOKEN=
+
+# API Secrets
+HEALTHCHECK_SECRET=
+CRON_SECRET=
+
+# Optional
+TITLE=
+
+# Node environment
+NODE_ENV=development