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/PlanPermission.php

@@ -0,0 +1,53 @@
+<?php
+
+namespace App\Models;
+
+use Illuminate\Database\Eloquent\Factories\HasFactory;
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
+
+class PlanPermission extends Model
+{
+    use HasFactory;
+
+    protected $fillable = [
+        'plan_id',
+        'plan_feature_id',
+        'permission',
+        'is_granted',
+        'conditions',
+        'applies_during_trial',
+        'trial_permission_override',
+    ];
+
+    protected $casts = [
+        'is_granted' => 'boolean',
+        'applies_during_trial' => 'boolean',
+        'trial_permission_override' => 'boolean',
+        'conditions' => 'array',
+    ];
+
+    public function plan(): BelongsTo
+    {
+        return $this->belongsTo(Plan::class);
+    }
+
+    public function planFeature(): BelongsTo
+    {
+        return $this->belongsTo(PlanFeature::class);
+    }
+
+    public function scopeGranted($query)
+    {
+        return $query->where('is_granted', true);
+    }
+
+    public function isEffectivePermission(bool $isOnTrial = false): bool
+    {
+        if ($isOnTrial && $this->applies_during_trial && $this->trial_permission_override !== null) {
+            return $this->trial_permission_override;
+        }
+
+        return $this->is_granted;
+    }
+}