Credits System + Activation Campaign + /admin/users

• credits table: API call credits purchasable via Stripe 5-pack ($X for 2,500 calls) or granted manually
• Stripe webhook branch for metadata.product === "5pack" automatically grants credits on checkout
• activation_emails table tracks drip campaign sends and conversion events per recipient
• /admin/users shows id, email, api_calls_total, has_credits, credits_remaining for targeting