feat: Allow pausing and resuming active campaigns

[sahare77]

Description
Once a campaign is launched, there is no UI to pause it. If a user realizes there's a typo, they cannot stop the remaining emails from going out.

Steps to Reproduce

1. Launch a campaign.
2. Go to the Campaigns dashboard.
3. Try to find a Pause button.

Expected Behavior
Add a "Pause" button next to active campaigns. This should update the campaign status to PAUSED. The Celery beat scheduler should check this status before processing the next batch.

Implementation Hints

# backend/campaigns/models.py
# Ensure Celery checks campaign status
active_campaigns = Campaign.objects.filter(status='ACTIVE')

[sahare77]

Hello @Kuldeeep18, I want to work on this issue. Please assign it to me under GSSoC 2026.

[aaniya22]

pls assign this to me under gssoc

[sahare77]

/assign under gssoc'26