Integrate SendGrid for organization welcome/thank-you emails (replace Firebase mail sender)

[yuvraj-shishodia]

Description

Currently, welcome/thank-you emails sent after successful organization creation
are queued via Firebase’s mail collection. We want to replace this with a direct
SendGrid integration so emails are sent reliably via SendGrid instead of relying
on the Firebase email extension/service.

This issue tracks implementing SendGrid as the primary email provider for the
organization creation thank-you email flow.

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Current Behavior

• After creating an organization, a thank-you/welcome email is queued via
  Firebase’s mail collection
• Delivery depends on the Firebase email extension or a custom mail listener
• Email provider configuration is indirect and harder to monitor

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Expected Behavior

• After successful organization creation, a thank-you/welcome email is sent via
  SendGrid
• Firebase mail collection is no longer used for this specific flow
• Email delivery status can be monitored via SendGrid logs/dashboard
• Failures are handled gracefully (retry/logging)

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Scope

Backend

• Add SendGrid client configuration (API key via env/secret)
• Implement SendGrid send helper for organization welcome emails
• Replace Firebase mail enqueue logic for this flow with SendGrid send call
• Add basic error handling and logging for send failures

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Email Template

• Migrate the existing welcome/thank-you email template to SendGrid format
• Support subject + HTML/text body
• Ensure personalization fields (org name, user name, etc.) are preserved

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Configuration & Security

• Store SendGrid API key securely (env/secret manager)
• Ensure keys are not committed to the repo
• Add environment-specific configuration (dev/staging/prod)

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Acceptance Criteria

• Organization creation triggers a SendGrid email successfully
• Firebase mail collection is no longer used for this flow
• Email content matches the existing welcome/thank-you template
• SendGrid failures are logged and do not break org creation
• Configuration works in dev/staging/prod

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Out of Scope

• Migrating other email flows to SendGrid (tracked separately if needed)
• Bulk email or marketing emails

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Notes

• Confirm SendGrid account, sender identity, and domain authentication
• Product may want to review email content before final rollout