collab: Add token_spend_in_cents column to billing_subscriptions table (#39404)

This PR adds a `token_spend_in_cents` and associated
`token_spend_in_cents_updated_at` column to the `billing_subscriptions`
table.

Release Notes:

- N/A

crates/collab/migrations/20251002214229_add_token_spend_in_cents_to_billing_subscriptions.sql

@@ -0,0 +1,3 @@
+alter table billing_subscriptions
+    add column token_spend_in_cents integer,
+    add column token_spend_in_cents_updated_at timestamp without time zone;