feat: add lead activity timeline

[saurabhhhcodes]

Closes #389

What changed

• Added a lead timeline endpoint that merges lead creation, campaign enrollment, send, open, click, reply, and bounce events into one chronological payload.
• Added a right-side timeline drawer on the Leads page that opens when a lead row is clicked.
• Recorded send and bounce timestamps on CampaignLead so the timeline can show real event times.
• Fixed the lead delete-all endpoint to delete from the org-scoped base queryset instead of a distinct queryset.

Validation

• python3 -m py_compile backend/leads/views.py backend/campaigns/models.py backend/campaigns/tasks.py backend/campaigns/views.py backend/leads/tests.py backend/campaigns/tests.py
• python3 backend/manage.py test leads.tests.LeadTimelineTests leads.tests.LeadIsolationAPITests campaigns.tests.CampaignWorkflowTests.test_send_email_step_records_sent_timestamp_for_mock_delivery campaigns.tests.CampaignWorkflowTests.test_email_webhook_persists_bounce_metadata -v 2
• node --check /tmp/leads-inline.js
• git diff --check

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