Allow Direct Subscription Creation via API

[Anthonykung]

Summary:
It would be super helpful if Polar.sh could support direct subscription creation through the API, without requiring a customer-facing checkout session.

Description:
Currently, all subscriptions must be created using the checkout flow. This makes sense for most user scenarios, but there are cases where programmatically creating a subscription is needed. For example:

• Automate plan adjustments, such as applying discounts or price changes
• Migrating subscription for any number of reasons, such as custom prorate logic for plan change or reset subscription renewal date
• Offering staff/admin-created comped plans or custom trials
• Automated workflows where user-facing checkout isn't appropriate

Stripe and PayPal both offer this capability, allowing subscriptions to be created (and fully managed) via backend scripts or admin tools. This enables much greater flexibility for automation, admin tools, and integrations.

Why This Matters:
Adding this feature would help us automate migrations, admin comp subscriptions, and build a smoother experience for both users and staff. It would also bring Polar.sh in line with similar platforms like Stripe and PayPal.

Thank you for considering!