feat: implement comprehensive multi-provider payment processing system

- Add unified payment provider architecture with contract-based design
  - Implement 6 payment providers: Stripe, Lemon Squeezy, Polar, Oxapay, Crypto, Activation Keys
  - Create subscription management with lifecycle handling (create, cancel, pause, resume, update)
  - Add coupon system with usage tracking and trial extensions
  - Build Filament admin resources for payment providers, subscriptions, coupons, and trials
  - Implement payment orchestration service with provider registry and configuration management
  - Add comprehensive payment logging and webhook handling for all providers
  - Create customer analytics dashboard with revenue, churn, and lifetime value metrics
  - Add subscription migration service for provider switching
  - Include extensive test coverage for all payment functionality

app/Models/SubscriptionChange.php

@@ -0,0 +1,138 @@
+<?php
+
+namespace App\Models;
+
+use Illuminate\Database\Eloquent\Model;
+
+class SubscriptionChange extends Model
+{
+    protected $fillable = [
+        'subscription_id',
+        'user_id',
+        'change_type',
+        'change_description',
+        'old_values',
+        'new_values',
+        'reason',
+        'effective_at',
+        'processed_at',
+        'is_processed',
+        'metadata',
+    ];
+
+    protected $casts = [
+        'old_values' => 'array',
+        'new_values' => 'array',
+        'effective_at' => 'datetime',
+        'processed_at' => 'datetime',
+        'is_processed' => 'boolean',
+        'metadata' => 'array',
+    ];
+
+    protected $dates = [
+        'effective_at',
+        'processed_at',
+    ];
+
+    /**
+     * Relationships
+     */
+    public function subscription()
+    {
+        return $this->belongsTo(Subscription::class);
+    }
+
+    public function user()
+    {
+        return $this->belongsTo(User::class);
+    }
+
+    /**
+     * Get human-readable change type
+     */
+    public function getChangeTypeLabelAttribute(): string
+    {
+        return [
+            'plan_change' => 'Plan Change',
+            'cancellation' => 'Cancellation',
+            'pause' => 'Pause',
+            'resume' => 'Resume',
+            'migration' => 'Migration',
+            'provider_change' => 'Provider Change',
+        ][$this->change_type] ?? ucfirst($this->change_type);
+    }
+
+    /**
+     * Mark as processed
+     */
+    public function markAsProcessed(): void
+    {
+        $this->update([
+            'is_processed' => true,
+            'processed_at' => now(),
+        ]);
+    }
+
+    /**
+     * Scope: By change type
+     */
+    public function scopeByType($query, string $type)
+    {
+        return $query->where('change_type', $type);
+    }
+
+    /**
+     * Scope: Processed
+     */
+    public function scopeProcessed($query)
+    {
+        return $query->where('is_processed', true);
+    }
+
+    /**
+     * Scope: Pending processing
+     */
+    public function scopePending($query)
+    {
+        return $query->where('is_processed', false);
+    }
+
+    /**
+     * Scope: By user
+     */
+    public function scopeByUser($query, $userId)
+    {
+        return $query->where('user_id', $userId);
+    }
+
+    /**
+     * Scope: Within date range
+     */
+    public function scopeBetweenDates($query, $startDate, $endDate)
+    {
+        return $query->whereBetween('effective_at', [$startDate, $endDate]);
+    }
+
+    /**
+     * Create a subscription change record
+     */
+    public static function createRecord(
+        Subscription $subscription,
+        string $changeType,
+        string $description,
+        ?array $oldValues = null,
+        ?array $newValues = null,
+        ?string $reason = null
+    ): self {
+        return static::create([
+            'subscription_id' => $subscription->id,
+            'user_id' => $subscription->user_id,
+            'change_type' => $changeType,
+            'change_description' => $description,
+            'old_values' => $oldValues,
+            'new_values' => $newValues,
+            'reason' => $reason,
+            'effective_at' => now(),
+        ]);
+    }
+}