Implement complete PostHog plugin foundation with collection tracking and admin login events

[Copilot]

This PR implements a complete Payload CMS plugin that integrates with PostHog analytics, providing automatic tracking of collection changes and admin login events.

Features Implemented

🚀 Plugin Foundation

• Complete TypeScript plugin scaffolding following Payload conventions
• NPM package structure with proper build and type generation
• Environment-based configuration with sensible defaults

📊 Collection Event Tracking

Automatically tracks all collection operations across your Payload CMS:

• Create events: {collection_slug}_created
• Update events: {collection_slug}_updated
• Delete events: {collection_slug}_deleted

Each event includes:

{
  id: string,           // Document ID
  userId: string | null, // User performing the action
  data: object,         // Full document data
  timestamp: string     // ISO timestamp
}

🔐 Admin Login Tracking

Monitors admin user authentication with automatic user identification:

• Event: admin_login
• Properties: { userId, email, loginTime, timestamp }
• Automatically identifies users in PostHog for enhanced analytics

⚙️ Configuration

Simple setup via environment variables:

# Required
POSTHOG_PROJECT_API_KEY=your_project_api_key

# Optional (defaults to EU instance)
POSTHOG_HOST=https://app.posthog.com

🛡️ Error Handling

• Graceful degradation when PostHog is not configured
• Comprehensive error logging without breaking your application
• Safe try-catch blocks around all PostHog operations

Usage Example

import { buildConfig } from 'payload/config'
import { posthogPlugin } from 'payload-plugin-posthog'

export default buildConfig({
  // ... your existing config
  plugins: [
    posthogPlugin({
      // Optional: override environment variables
      // apiKey: 'your_api_key',
      // host: 'https://app.posthog.com'
    })
  ]
})

Development Setup

• Includes /dev folder with example Payload configuration
• Complete TypeScript setup with type definitions
• Ready for npm publication

The plugin automatically hooks into all collections and auth-enabled collections without requiring any additional configuration, making it a true "plug and play" solution for PostHog analytics in Payload CMS.

Fixes #1.

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