feat: implement comprehensive enhanced plan management system

- Create 7 new models with full relationships and business logic:
     * PlanFeature: Define available features with categories and types
     * PlanFeatureLimit: Manage usage limits per plan with trial overrides
     * PlanPermission: Granular permissions system for features
     * PlanProvider: Multi-provider payment configuration
     * PlanTier: Hierarchical plan structure with upgrade paths
     * PlanUsage: Real-time usage tracking and analytics
     * TrialConfiguration: Advanced trial settings per plan

   - Enhance Plan model with 25+ new methods:
     * Feature checking: hasFeature(), canUseFeature(), getRemainingUsage()
     * Permission system: hasPermission() with trial support
     * Payment providers: getAllowedProviders(), supportsProvider()
     * Trial management: hasTrial(), getTrialConfig()
     * Upgrade paths: isUpgradeFrom(), getUpgradePath()
     * Utility methods: getBillingCycleDisplay(), metadata handling

   - Completely redesign PlanResource with tabbed interface:
     * Basic Info: Core plan configuration with dynamic billing cycles
     * Features & Limits: Dynamic feature management with trial overrides
     * Payment Providers: Multi-provider configuration (Stripe, Lemon Squeezy, etc.)
     * Trial Settings: Advanced trial configuration with always-visible toggle

   - Create new Filament resources:
     * PlanFeatureResource: Manage available features by category
     * PlanTierResource: Hierarchical tier management with parent-child relationships

   - Implement comprehensive data migration:
     * Migrate legacy plan data to new enhanced system
     * Create default features (mailbox accounts, email forwarding, etc.)
     * Preserve existing payment provider configurations
     * Set up trial configurations (disabled for legacy plans)
     * Handle duplicate data gracefully with rollback support

   - Add proper database constraints and indexes:
     * Unique constraints on plan-feature relationships
     * Foreign key constraints with cascade deletes
     * Performance indexes for common queries
     * JSON metadata columns for flexible configuration

   - Fix trial configuration form handling:
     * Add required validation for numeric fields
     * Implement proper null handling with defaults
     * Add type casting for all numeric fields
     * Ensure database constraint compliance

app/Models/PlanProvider.php

@@ -0,0 +1,55 @@
+<?php
+
+namespace App\Models;
+
+use Illuminate\Database\Eloquent\Factories\HasFactory;
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
+
+class PlanProvider extends Model
+{
+    use HasFactory;
+
+    protected $fillable = [
+        'plan_id',
+        'provider',
+        'provider_price_id',
+        'provider_variant_id',
+        'provider_product_id',
+        'is_enabled',
+        'price',
+        'currency',
+        'provider_data',
+        'sort_order',
+    ];
+
+    protected $casts = [
+        'is_enabled' => 'boolean',
+        'price' => 'decimal:2',
+        'provider_data' => 'array',
+    ];
+
+    public function plan(): BelongsTo
+    {
+        return $this->belongsTo(Plan::class);
+    }
+
+    public function scopeEnabled($query)
+    {
+        return $query->where('is_enabled', true);
+    }
+
+    public function scopeByProvider($query, string $provider)
+    {
+        return $query->where('provider', $provider);
+    }
+
+    public function getProviderData(?string $key = null, $default = null)
+    {
+        if ($key) {
+            return data_get($this->provider_data, $key, $default);
+        }
+
+        return $this->provider_data;
+    }
+}