🛂 Add new yearly plans and graduated pricing

BREAKING CHANGE: Stripe environment variables have changed. New ones are required. Check out the new Stripe configuration in the
docs.

Closes #457

packages/schemas/features/billing/subscription.ts

@@ -1,9 +1,9 @@
 import { z } from 'zod'
 
 export const subscriptionSchema = z.object({
-  additionalChatsIndex: z.number(),
-  additionalStorageIndex: z.number(),
+  isYearly: z.boolean(),
   currency: z.enum(['eur', 'usd']),
+  cancelDate: z.date().optional(),
 })
 
 export type Subscription = z.infer<typeof subscriptionSchema>