fix: resolved race condition for get certificate button

[cap-code]

1. Add server-side locking (SELECT ... FOR UPDATE) in create_certificate to serialize concurrent requests for the same user+course, preventing duplicate certificates from being created when the "Get Certificate" button is clicked rapidly

2. Add a composite unique index on (member, course) via on_doctype_update as a database-level safety net, with a DuplicateEntryError handler that gracefully returns the existing certificate.

3. Add a frontend loading guard and disable the "Get Certificate" button while a request is in flight to prevent unnecessary duplicate submissions.

FIXES #2408

[raizasafeel]

Could you go through contribution guidelines? We want to ideally use direct sql as little as possible. Could you check out alternatives such as frappe.db.add_unique or simply look into frappe query builder?

For the duplicates: the real fix is a DB-level unique constraint on member, course (think frappe.db.add_unique()), since that's what actually stops two simultaneous request from both creating a certificate (an if exists check alone can't, because both requests pass the check before either inserts). Keep an if certificate exists, return it as a fast path, but also wrap the insert so that if the unique constraint trips on a race, you catch it and return the existing one instead of erroring.

We'll also need a data patch here, the unique constraint won't apply on sites that already have duplicate certificates. Since that one deletes data, I'm happy to add it as a co author once your part is in.

Also could you resolve the conflicts (with develop branch) and add a simple test (insert a certificate, attempt a second for the same member+course, assert it's rejected / count stays 1)?

Let me know if you need any help with it. Thanks for the contribution.