Cold Outreach + UTM Signup Attribution

• Wave 2 cold outreach batch to Vestiaire/Grailed/Depop engineering contacts via Postmark
• cold_outreach_contacts table: contact upsert, status tracking, UTM attribution per contact
• UTM middleware captures utm_source/medium/campaign/content on every request; stored on user at signup
• GET /admin/conversions: UTM attribution dashboard showing which channels convert to signups