feat: add plan_prices table, PlanPrice model, and Plan relationship

- Create plan_prices migration with unique(plan_id, billing_cycle)
- PlanPrice model with factory
- Plan::prices() relationship and priceForCycle() helper
- Tests for model relationships, uniqueness, cascade delete

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

website/app/Models/Plan.php

@@ -49,6 +49,16 @@ class Plan extends Model
         return $this->hasMany(Service::class);
     }
 
+    public function prices(): HasMany
+    {
+        return $this->hasMany(PlanPrice::class);
+    }
+
+    public function priceForCycle(string $cycle): ?PlanPrice
+    {
+        return $this->prices()->where('billing_cycle', $cycle)->first();
+    }
+
     public function isAvailable(): bool
     {
         if ($this->status !== 'active') {

website/app/Models/PlanPrice.php

@@ -0,0 +1,33 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Models;
+
+use Illuminate\Database\Eloquent\Factories\HasFactory;
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
+
+class PlanPrice extends Model
+{
+    use HasFactory;
+
+    protected $fillable = [
+        'plan_id',
+        'billing_cycle',
+        'price',
+        'stripe_price_id',
+    ];
+
+    protected function casts(): array
+    {
+        return [
+            'price' => 'decimal:2',
+        ];
+    }
+
+    public function plan(): BelongsTo
+    {
+        return $this->belongsTo(Plan::class);
+    }
+}