moved invoice pdf generation off the request thread into a background job. users get an email with the link when it's ready. big invoices were timing out at 30s. added retry 3x. tested locally with a 400 page invoice and on staging

Summary
This PR moves the invoice PDF generation off the request thread into a background job to improve performance and prevent timeouts for large invoices.

Changes

• Users receive an email with a link to the invoice when it's ready.
• Increased timeout handling by adding a retry mechanism of 3 attempts.

Testing

• Tested locally with a 400-page invoice and on staging.