fix(checkout-links): Prevent auto-escaping {CHECKOUT_ID} in success URL

[Dhanus3133]

Summary

Fixes an issue where {CHECKOUT_ID} in the success URL was auto-escaped to %7BCHECKOUT_ID%7D after saving a checkout link.

Why This Change Was Made

• Libraries like pypandic follow RFC 3986 and escape {} as %7B/%7D.
• This ensures the placeholder is kept as-is and replaced correctly when generating the final URL.

Fixes: #6300

[vercel]

@Dhanus3133 is attempting to deploy a commit to the polar-sh Team on Vercel.

A member of the Team first needs to authorize it.

[frankie567]

Thank you for having a look into this, but I don't think this is the right approach: calling unquote on the whole URL opens a security risk, as everything can be decoded, even malicious payloads and stuff like that.

IMO, we should do the following:

• Find a way to "unescape" {CHECKOUT_ID} (and only that) when creating/updating that URL. We can probably rely on a custom Pydantic validator for this. Don't forget to use it both in checkout/schemas.py and checkout_link/schemas.py.
• Create a database migration to unescape {CHECKOUT_ID} (and only that) from existing checkout links.