feat: add celery beat health check

[saurabhhhcodes]

Adds a periodic Celery Beat heartbeat and a small authenticated health endpoint so the dashboard can detect when scheduled background processing stops updating.

What changed:

• added a celery_heartbeat task that writes a timestamp into cache every minute
• added /api/v1/health/celery-beat/ to report whether the heartbeat is fresh enough
• configured cache backends so the heartbeat works locally and can use Redis in non-debug deployments
• added regression tests for healthy, stale, and missing heartbeat states
• added the campaigns migration merge file needed to keep the test database migratable on this baseline

Validation:

• python3 -m py_compile backend/backend/settings.py backend/campaigns/tasks.py backend/campaigns/views.py backend/backend/urls.py backend/campaigns/tests.py backend/campaigns/migrations/0010_merge_0009_campaign_cached_counters_0009_campaignlead_bounce_metadata.py
• python3 backend/manage.py test campaigns.tests.CampaignWorkflowTests.test_celery_beat_health_reports_503_until_heartbeat_updates campaigns.tests.CampaignWorkflowTests.test_celery_beat_health_reports_healthy_after_recent_heartbeat campaigns.tests.CampaignWorkflowTests.test_celery_beat_health_marks_stale_heartbeat_unhealthy -v 2
• git diff --check

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