Replace SendGrid SMTP with Resend for transactional email

[dadachi]

Summary

• Replace SendGrid SMTP delivery with Resend API-based email delivery
• Simplify mail initializer by removing SMTP configuration in favor of Resend's Rails integration
• Update credentials template to use Resend API key instead of SMTP settings
• Add resend gem dependency

Test plan

• Verify email delivery works in staging/production with Resend
• Confirm credentials template generates correct structure for new environments

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