collab: Add billing_subscriptions table (#15448)

This PR adds a new `billing_subscriptions` table to the database, as
well as some accompanying models/queries.

In this table we store a minimal amount of data from Stripe:

- The Stripe customer ID
- The Stripe subscription ID
- The status of the Stripe subscription

This should be enough for interactions with the Stripe API (e.g., to
[create a customer portal
session](https://docs.stripe.com/api/customer_portal/sessions/create)),
as well as determine whether a subscription is active (based on the
`status`).

Release Notes:

- N/A

crates/collab/migrations/20240729170526_add_billing_subscription.sql

@@ -0,0 +1,12 @@
+CREATE TABLE IF NOT EXISTS billing_subscriptions (
+    id SERIAL PRIMARY KEY,
+    created_at TIMESTAMP WITHOUT TIME ZONE NOT NULL DEFAULT now(),
+    user_id INTEGER NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    stripe_customer_id TEXT NOT NULL,
+    stripe_subscription_id TEXT NOT NULL,
+    stripe_subscription_status TEXT NOT NULL
+);
+
+CREATE INDEX "ix_billing_subscriptions_on_user_id" ON billing_subscriptions (user_id);
+CREATE INDEX "ix_billing_subscriptions_on_stripe_customer_id" ON billing_subscriptions (stripe_customer_id);
+CREATE UNIQUE INDEX "uix_billing_subscriptions_on_stripe_subscription_id" ON billing_subscriptions (stripe_subscription_id);