fix: update CACHE_DIR for vercel

[Chaitanya-Keyal]

Description

This PR updates the vercel branch to use a cache directory compatible with Vercel’s serverless runtime.

What changed

• In api/cache.php, changed the cache directory from a project-relative path to /tmp/cache.
• This avoids mkdir(): Read-only file system warnings when deploying on Vercel, since the project filesystem is read-only and /tmp is the only writable location.

Why

After the recent cache update, the following warnings showed up on my Vercel deployment due to which the card stopped rendering properly on my github readme:

Warning: mkdir(): Read-only file system in /var/task/user/api/cache.php on line 57

Warning: http_response_code(): Cannot set response code - headers already sent (output started at /var/task/user/api/cache.php:57) in /var/task/user/api/card.php on line 876

Warning: Cannot modify header information - headers already sent by (output started at /var/task/user/api/cache.php:57) in /var/task/user/api/card.php on line 877

• Vercel serverless functions run on a read-only filesystem; attempts to create or write to directories inside the project path fail and cause PHP warnings.
• Using /tmp/cache allows the file-based cache to work during a function’s lifetime without requiring any additional configuration from users.

Notes

• This change is scoped to the vercel branch only.
• If needed in the future, the cache directory could be made configurable via an environment variable (for example, CACHE_DIR), but this PR keeps the fix minimal.

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• Tested locally with a valid username
• Tested locally with an invalid username
• Ran tests with composer test
• Added or updated test cases to test new features
• Tested on Vercel deployment

Checklist

• I have checked to make sure no other pull requests are open for this issue
• The code is properly formatted and is consistent with the existing code style
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings