fix(api): implement pagination and bounded concurrency for broadcast …

[VaishnaviL2005]

🛑 STOP: Assignment & File Scope Check

• I am assigned to this issue.
• I verified that this PR ONLY touches the required files.

📋 PR Summary & Link

• **Closes [BUG] Notification broadcast route truncates recipients at Supabase max_rows limit #1905 **
• Implemented Offset Pagination: Fixes broadcast recipient truncation caused by Supabase max_rows limits by introducing paginated subscriber retrieval using .range() and explicit .order("id").
• Added Bounded-Concurrency: Additionally improves delivery throughput by processing recipients in chunks of 50, respecting external SMS/WhatsApp provider rate limits without sequential blocking.

📸 Proof of Work (Screenshots / Logs)

🏷️ PR Type

• 🐛 type: bug
• ✨ type: feature
• 📖 type: docs
• 🧪 type: testing
• 🔒 type: security
• ⚡ type: performance
• 🎨 type: design
• ♻️ type: refactor
• 🛠️ type: devops
• ♿ type: accessibility

✅ Checklist

• My PR has a linked issue (Closes #1905 )
• I have pulled the latest main and resolved any conflicts

[github-actions]

🎉 Congratulations @VaishnaviL2005! Your Pull Request "fix(api): implement pagination and bounded concurrency for broadcast …" has been successfully merged by @dipexplorer.

Thank you for your valuable contribution to SahiDawa! 🇮🇳
If this was for GSSoC 2026, your work is officially merged and valid. Keep up the great work and feel free to claim other open issues. 🚀

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