feat(plugins): Implement modular plugin system with self-registering events

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This implements a clean plugin architecture based on PR #4255 discussion:

Core Components:
- PluginInterface: Standard contract all plugins must implement
- BasePlugin: Abstract class with common functionality
- PluginManager: Discovers and loads plugins from app/Plugins/
- Plugin_config: Model for plugin settings storage

Architecture:
- Each plugin registers its own event listeners via registerEvents()
- No hardcoded plugin dependencies in core Events.php
- Generic event triggers (item_sale, item_change, etc.) remain in core code
- Plugins can be enabled/disabled via database settings
- Clean separation: plugin orchestrators vs MVC components

Example Implementations:
- ExamplePlugin: Simple plugin demonstrating event logging
- MailchimpPlugin: Integration with Mailchimp for customer sync

Admin UI:
- Plugin management controller at Controllers/Plugins/Manage.php
- Plugin management view at Views/plugins/manage.php

Database:
- ospos_plugin_config table for plugin settings (key-value store)
- Migration creates table with timestamps

Documentation:
- Comprehensive README with architecture patterns
- Simple vs complex plugin examples
- MVC directory structure guidance

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