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.

supabase/seed.sql

@@ -0,0 +1,37 @@
+-- These webhooks are only for development purposes.
+-- In production, you should manually create webhooks in the Supabase dashboard (or create a migration to do so).
+-- We don't do it because you'll need to manually add your webhook URL and secret key.
+
+-- this webhook will be triggered after every insert on the accounts_memberships table
+create trigger "accounts_memberships_insert" after insert
+on "public"."accounts_memberships" for each row
+execute function "supabase_functions"."http_request"(
+  'http://localhost:3000/api/database/webhook',
+  'POST',
+  '{"Content-Type":"application/json", "X-Supabase-Event-Signature":"WEBHOOKSECRET"}',
+  '{}',
+  '1000'
+);
+
+-- this webhook will be triggered after every insert on the accounts_memberships table
+create trigger "account_membership_delete" after insert
+on "public"."accounts_memberships" for each row
+execute function "supabase_functions"."http_request"(
+  'http://localhost:3000/api/database/webhook',
+  'POST',
+  '{"Content-Type":"application/json", "X-Supabase-Event-Signature":"WEBHOOKSECRET"}',
+  '{}',
+  '1000'
+);
+
+-- this webhook will be triggered after a delete on the subscriptions table
+-- which should happen when a user deletes their account (and all their subscriptions)
+create trigger "account_delete" after delete
+on "public"."subscriptions" for each row
+execute function "supabase_functions"."http_request"(
+  'http://localhost:3000/api/database/webhook',
+  'POST',
+  '{"Content-Type":"application/json", "X-Supabase-Event-Signature":"WEBHOOKSECRET"}',
+  '{}',
+  '1000'
+);