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

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

app/Plugins/ExamplePlugin.php

@@ -0,0 +1,110 @@
+<?php
+
+namespace App\Plugins;
+
+use App\Libraries\Plugins\BasePlugin;
+use CodeIgniter\Events\Events;
+
+/**
+ * Example Plugin
+ * 
+ * A simple example plugin demonstrating the plugin system.
+ * Logs events to the debug log for demonstration purposes.
+ */
+class ExamplePlugin extends BasePlugin
+{
+    public function getPluginId(): string
+    {
+        return 'example';
+    }
+
+    public function getPluginName(): string
+    {
+        return 'Example Plugin';
+    }
+
+    public function getPluginDescription(): string
+    {
+        return 'A demonstration plugin that logs OSPOS events. This plugin shows how plugins can integrate with the system.';
+    }
+
+    public function getVersion(): string
+    {
+        return '1.0.0';
+    }
+
+    public function registerEvents(): void
+    {
+        Events::on('item_change', [$this, 'onItemChange']);
+        Events::on('item_sale', [$this, 'onItemSale']);
+        
+        $this->log('debug', 'Example plugin events registered');
+    }
+
+    public function install(): bool
+    {
+        $this->log('info', 'Installing Example Plugin');
+        
+        $this->setSetting('log_changes', '1');
+        $this->setSetting('log_sales', '1');
+        
+        return true;
+    }
+
+    public function uninstall(): bool
+    {
+        $this->log('info', 'Uninstalling Example Plugin');
+        return true;
+    }
+
+    public function getConfigView(): ?string
+    {
+        return 'Plugins/example/config';
+    }
+
+    public function getSettings(): array
+    {
+        return [
+            'log_changes' => $this->getSetting('log_changes', '1'),
+            'log_sales' => $this->getSetting('log_sales', '1'),
+        ];
+    }
+
+    public function saveSettings(array $settings): bool
+    {
+        if (isset($settings['log_changes'])) {
+            $this->setSetting('log_changes', $settings['log_changes'] ? '1' : '0');
+        }
+        
+        if (isset($settings['log_sales'])) {
+            $this->setSetting('log_sales', $settings['log_sales'] ? '1' : '0');
+        }
+        
+        return true;
+    }
+
+    /**
+     * Handle item change event.
+     */
+    public function onItemChange(int $itemId): void
+    {
+        if (!$this->isEnabled() || $this->getSetting('log_changes', '1') !== '1') {
+            return;
+        }
+        
+        $this->log('info', "Item changed: ID {$itemId}");
+    }
+
+    /**
+     * Handle item sale event.
+     */
+    public function onItemSale(array $saleData): void
+    {
+        if (!$this->isEnabled() || $this->getSetting('log_sales', '1') !== '1') {
+            return;
+        }
+        
+        $saleId = $saleData['sale_id_num'] ?? 'unknown';
+        $this->log('info', "Item sale: ID {$saleId}");
+    }
+}