Build correct in-app pricing (Pro, Business, Agency) from new website and sync with Stripe

Align the in-app pricing with the new marketing website and wire it through to Stripe.

## Goal

Implement the correct, current pricing tiers in the app to match the new website: Pro, Business, and Agency. Ensure each plan's prices, limits, and entitlements reflect the website, and that the new pricing is synced with Stripe.

## Scope

* Define the canonical tiers (Pro, Business, Agency) with their prices (monthly/annual) and feature limits as published on the new website.
* Update the app's plan/pricing configuration and any plan-gating logic (seats, limits, entitlements) to match.
* Create/update the corresponding Stripe Products and Prices and connect them to the app (checkout, subscription management, webhooks).
* Verify checkout, upgrades/downgrades, and billing portal reflect the new tiers.

## Notes

* Confirm exact numbers and feature matrix against the new website before implementing.
* Coordinate with existing billing code (stripe.ts, seatLimits.ts) and plan handling.

## Acceptance

* Pro, Business, and Agency are available and correctly priced in the app, matching the website.
* Stripe products/prices exist and are linked; checkout and subscription updates work end-to-end.