refactor: restructure Stripe pricing config to support monthly and annual tiers

- Nest price IDs under `monthly` and `annual` keys for each tier (basic, plus, pro)

config/services.php

@@ -74,9 +74,18 @@ return [
 
     'stripe' => [
         'prices' => [
-            'basic' => env('STRIPE_PRICE_BASIC'),
-            'plus' => env('STRIPE_PRICE_PLUS'),
-            'pro' => env('STRIPE_PRICE_PRO'),
+            'basic' => [
+                'monthly' => env('STRIPE_PRICE_BASIC_MONTHLY'),
+                'annual' => env('STRIPE_PRICE_BASIC_ANNUAL'),
+            ],
+            'plus' => [
+                'monthly' => env('STRIPE_PRICE_PLUS_MONTHLY'),
+                'annual' => env('STRIPE_PRICE_PLUS_ANNUAL'),
+            ],
+            'pro' => [
+                'monthly' => env('STRIPE_PRICE_PRO_MONTHLY'),
+                'annual' => env('STRIPE_PRICE_PRO_ANNUAL'),
+            ],
         ],
     ],