feat: mass email campaign system with CSV upload

[indrasuthar07]

Description

• a complete bulk email campaign system with CSV upload, batch-based sending, and real-time progress tracking.
• -Includes backend support for campaign storage, batch processing, personalized templates, and tracking pixels.
• Frontend provides CSV preview, progress bar, and live status updates. Fully configurable via environment variables.

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Semver Changes

• Patch (bug fix, no new features)
• Minor (new features, no breaking changes)
• Major (breaking changes)

Issues

closes #5

List any issues that this pull request closes.

Checklist

• I have read the Contributing Guidelines.

[MD-IRFAN-RAJ]

@indrasuthar07 did you update the .env.example?