Email provider integration

[Alaa2019-ml]

I used a Gmail account for StudyBridge because creating a custom domain email requires buying a domain and potentially paying for email hosting. As the project is still in its early stage, Gmail was a free and practical solution.

[Copilot]

Pull request overview

This PR adds production email provider integration using Gmail SMTP. Previously, the email service only supported development mode with Ethereal test accounts and threw an error in production. The changes enable the application to send real emails in production using configurable SMTP settings.

Changes:

• Added production SMTP configuration with environment variable support (SMTP_HOST, SMTP_PORT, SMTP_SECURE, SMTP_USER, SMTP_PASS)
• Created getRequiredEnv helper function for validating required environment variables
• Made email sender address configurable via EMAIL_FROM environment variable
• Refactored transporter initialization to separate production and development logic

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