feat: brand unsubscribe page with org settings

[saurabhhhcodes]

What changed

• Added organization-level branding fields for unsubscribe pages: title, message, and logo URL.
• Updated the public unsubscribe response to render a branded confirmation/unsubscribe page using the lead's organization.
• Added a test that verifies custom branding is rendered in the HTML response.
• Added the migration-graph merge migration needed to keep Django's migration path linear.

Why

• The public unsubscribe page was hardcoded and could not reflect the organization behind the campaign.
• This keeps the opt-out flow professional while preserving the existing compliance behavior.

Validation

• python3 -m py_compile backend/campaigns/views.py backend/tenants/models.py backend/campaigns/tests.py backend/tenants/migrations/0003_organization_unsubscribe_branding.py backend/campaigns/migrations/0010_merge_0009_campaign_cached_counters_0009_campaignlead_bounce_metadata.py
• git diff --check
• python3 backend/manage.py migrate --noinput
• Direct Django client check against /api/v1/unsubscribe/<lead>/<token>/ confirmed custom title, message, and logo output

Closes #403

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