Overage: buy extra 50,000-email blocks for $10 #127

Part of #126.

Goal

When an organization on Starter, Pro or Business reaches its monthly email limit, the owner can buy extra 50,000-email blocks for $10 each, instead of upgrading.

Work

• Limit: add a monthly email limit override to organization_plan_policies (there are teams and contacts overrides today, but none for emails). The effective monthly limit is the plan limit plus 50,000 × the blocks bought for the current period, applied in resolvePlanPolicy and enforced in reserveSend (marketing, transactional and outbound mail).
• Billing: a Dodo one-time product, $10 per block, added to the catalog as an add-on offer. Bought blocks apply to the current billing month only and reset with the email counter.
• Dashboard: when the limit is reached, offer "Buy 50,000 more emails" next to Upgrade on the Plan tab. Show bought blocks in the Monthly sends card.
• API, MCP, docs: the plan_limit_reached error says blocks can be bought; document the endpoint; update the pricing docs.

Acceptance criteria

• An organization at its email limit can buy a block with a Dodo test payment, and sending resumes straight away.
• The extra emails are gone in the next billing month.
• Free can't buy blocks.
• Tests cover the effective limit with 0, 1 and several blocks, and the monthly reset.