⚡ (billing) Automatic usage-based billing (#924)

BREAKING CHANGE: Stripe environment variables simplified. Check out the
new configs to adapt your existing system.

Closes #906





<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
### Summary by CodeRabbit

**New Features:**
- Introduced a usage-based billing system, providing more flexibility
and options for users.
- Integrated with Stripe for a smoother and more secure payment process.
- Enhanced the user interface with improvements to the billing,
workspace, and pricing pages for a more intuitive experience.

**Improvements:**
- Simplified the billing logic, removing additional chats and yearly
billing for a more streamlined user experience.
- Updated email notifications to keep users informed about their usage
and limits.
- Improved pricing and currency formatting for better clarity and
understanding.

**Testing:**
- Updated tests and specifications to ensure the reliability of new
features and improvements.

**Note:** These changes aim to provide a more flexible and user-friendly
billing system, with clearer pricing and improved notifications. Users
should find the new system more intuitive and easier to navigate.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/schemas/features/billing/subscription.ts

@@ -1,7 +1,10 @@
 import { z } from 'zod'
 
 export const subscriptionSchema = z.object({
-  isYearly: z.boolean(),
+  currentBillingPeriod: z.object({
+    start: z.date(),
+    end: z.date(),
+  }),
   currency: z.enum(['eur', 'usd']),
   cancelDate: z.date().optional(),
   status: z.enum(['active', 'past_due']),

packages/schemas/features/telemetry.ts

@@ -62,7 +62,6 @@ const subscriptionUpdatedEventSchema = workspaceEvent.merge(
     name: z.literal('Subscription updated'),
     data: z.object({
       plan: z.nativeEnum(Plan),
-      additionalChatsIndex: z.number(),
     }),
   })
 )