Add automatic bounce polling for Google and custom SMTP/IMAP mailboxes

[ramyacm23]

Summary

Closes #211

This PR adds automatic bounce detection for connected mailboxes.

Previously, bounce handling relied mainly on webhook events, which meant users sending through custom mailbox setups did not always get bounce updates. This change adds periodic mailbox polling so unread bounce notifications are parsed automatically and the matching campaign lead is marked as BOUNCED.

What Changed

• Added a periodic Celery task to scan connected inboxes for unread bounce messages
• Added bounce parsing logic to extract failed recipient addresses from mailbox messages
• Updated campaign lead handling so matched leads are marked as BOUNCED and removed from further sequence execution
• Marked processed bounce emails as read to avoid duplicate processing
• Added support for custom SMTP/IMAP mailbox accounts in addition to existing Google-connected accounts
• Added backend endpoints and settings UI support to create/remove custom mailbox accounts
• Kept webhook bounce handling aligned with the same shared bounce-marking logic

Files Updated

• backend/campaigns/tasks.py
• backend/campaigns/gmail_service.py
• backend/campaigns/mailbox_service.py
• backend/campaigns/models.py
• backend/campaigns/google_auth_views.py
• backend/campaigns/views.py
• backend/campaigns/tests.py
• backend/backend/settings.py
• backend/backend/urls.py
• frontend/settings.html
• backend/campaigns/migrations/0009_connectedemailaccount_custom_mailbox_fields.py

Why This Fix

This ensures bounce tracking works not only for webhook-based flows, but also for users who send through connected inboxes and custom SMTP/IMAP servers, improving automation and campaign hygiene.

Testing

Ran:

python manage.py test campaigns
python manage.py makemigrations --check --dry-run campaigns
Both completed successfully.

Notes
Google-connected mailbox polling remains supported
Custom mailbox support now includes:
SMTP sending
IMAP bounce polling
A production smoke test with a real custom mailbox is still recommended after deployment

[ramyacm23]

@Kuldeeep18 , check with the pr

[Kuldeeep18]

Hi @ramyacm23! Thank you for the contribution. There are currently merge conflicts in backend/campaigns/tests.py. Please resolve the conflicts so that we can merge your PR.

[ramyacm23]

@Kuldeeep18 , working on the conflicts. will raise a pr shortly.

[ramyacm23]

@Kuldeeep18 , kindly check with the pr

[Kuldeeep18]

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[ramyacm23]

@Kuldeeep18 , as its advanced the changes are quite complex . if the changes are made manually it results in merge conflicts, i will rise a pr shortly .