Paid overage: buy extra email blocks and extra teams instead of upgrading #126

Background

#60 set the hosted plans: Starter, Pro and Business, priced on emails and teams. It also included overage pricing, which wasn't built. Today every plan has hard caps: past the monthly email limit or the team limit, sending or creating a team is blocked and the customer is asked to upgrade (plan_limit_reached names the next plan up). See the decision comment on #60 and ADR 0003.

Overage from #60:

Free has no overage; past its limits the customer upgrades. Enterprise overage is by contract.

Subtasks

• Extra email blocks: 50,000 emails for $10 (sub-issue)
• Extra teams: $5/mo on Pro, $3/mo on Business (sub-issue)

Shared constraints

• Prices live in the Dodo catalog, not in code. Each add-on needs its own Dodo products and catalog entries, and a higher BILLING_CATALOG_REVISION.
• Limit changes go through organization_plan_policies (ADR 0002), not the billing tables. Read paid state with commercialState().
• Update the dashboard Plan tab, the REST API docs and MCP tools, and the self-hosting docs. OSS installs have no limits, so add-ons are cloud only.
• Keep schema changes compatible with the previous release.