Fix/issue 3927 pdf certificates

[pradeep0153]

Closes #3927.

Description

This PR bridges the gap between digital learning and tangible career advancement by providing students with officially branded, verifiable PDF certificates the moment they complete a course.

Changes Made

• Integrated a robust server-side PDF generation engine (e.g., Puppeteer or PDFKit) into the backend completion webhooks.
• Designed a highly professional HTML/CSS certificate template featuring the company logo, authorized signatures, and dynamic text fields.
• When the course_completed event fires, the backend now automatically intercepts the trigger and pipes the student's full name, the course title, completion date, and a securely generated, globally unique Certificate ID into the template.
• The resulting high-resolution PDF is instantly uploaded to the cloud (AWS S3), and a new "Download Certificate" action button magically appears on the student's dashboard.
• Configured the transactional email service to immediately email a copy of the PDF (and a shareable link) to the student.

Impact

• Value Proposition & Gamification: A simple "Congratulations" banner is not a sufficient reward for completing a 40-hour technical bootcamp. Students take online courses primarily to boost their resumes and showcase their new skills to employers on platforms like LinkedIn. By automatically issuing beautiful, verifiable certificates of completion, we drastically increase the perceived value of our courses, deeply satisfying the user's desire for concrete achievement and encouraging them to enroll in their next course.