Don't offer free trial to users who have previously subscribed

[gaidheal1]

Feature Request

Before offering a free trial on the upgrade page, check whether the user has had a previous Stripe subscription. If they have, do not show the trial offer.

Motivation

Free trials are intended for new users. A returning user who previously subscribed and cancelled should not be offered another trial — both to protect revenue and to maintain consistency with Stripe's trial eligibility model (Stripe itself prevents duplicate trials on the same customer in some configurations).

Proposed Behaviour

• When rendering the upgrade page, check the user's Stripe customer history for any previous subscription (active, cancelled, or expired)
• If a previous subscription exists: show the standard upgrade/payment flow without any trial offer or trial-related copy
• If no previous subscription exists: show the current free trial offer as normal

Implementation Notes

• Stripe's Customer object retains subscription history; a list of past subscriptions can be retrieved via the Stripe API (stripe.Subscription.list(customer=..., status='all'))
• The backend can expose a flag (e.g. has_previous_subscription) on the subscription status endpoint, computed once per request
• The frontend upgrade page reads this flag and conditionally renders trial vs. standard copy
• Consider caching this flag on the Player or payments model to avoid repeated Stripe API calls

Acceptance Criteria

• Backend detects whether the Stripe customer has any prior subscription (any status)
• A has_previous_subscription (or equivalent) flag is returned to the frontend
• Upgrade page hides trial offer and trial-specific copy when the flag is true
• Upgrade page shows trial offer as normal when the flag is false (new user)
• Existing tests cover both cases