Subscription tests

[qianl15]

This PR adds basic tests and linter for our subscription app.

• Configured linter rule to demonstrate the usage of DBOS recommended settings.
• Added unit tests to make sure our CORS setting is correct for our website.
• Added a staging test to make sure we can correctly upgrade/downgrade users when they subscribe/unsubscribe from Stripe. Whenever we add/cancel subscriptions, Stripe sends events to our webhook hosted on staging cluster, the webhook in turn talks to staging to enforce entitlements.
• Created new Github workflows for both tests. Staging test runs every 6 hours.
• Currently, we don't have a good solution to tell if a user is pro or free tier. I use "link db" to differentiate different tiers.

Next step:

• Add a user info endpoint in DBOS Cloud to return user subscription status.
• Use refresh token instead of our current login flow.