Improve and update billing flow

This commit updates various components in the billing flow due to a new schema that supports multiple line items per plan. The added flexibility rendered 'line-items-mapper.ts' redundant, which has been removed. Additionally, webhooks have been created for handling account membership insertions and deletions, as well as handling subscription deletions when an account is deleted. This message also introduces a new service to handle sending out invitation emails. Lastly, the validation of the billing provider has been improved for increased security and stability.

apps/web/app/api/database/webhook.ts

@@ -1,3 +0,0 @@
-export function POST(request: Request) {
-  console.log(request);
-}

apps/web/app/api/database/webhook/route.ts

@@ -0,0 +1,20 @@
+import { z } from 'zod';
+
+import { DatabaseWebhookHandlerService } from '@kit/database-webhooks';
+
+const webhooksSecret = z
+  .string({
+    description: `The secret used to verify the webhook signature`,
+  })
+  .min(1)
+  .parse(process.env.SUPABASE_DB_WEBHOOK_SECRET);
+
+export async function POST(request: Request) {
+  const service = new DatabaseWebhookHandlerService();
+
+  await service.handleWebhook(request, webhooksSecret);
+
+  return new Response(null, {
+    status: 200,
+  });
+}