Create real Stripe products/prices and reconcile pricing tiers

[ianpatrickhines]

Context

Checkout cannot succeed until real Stripe products/prices exist and their IDs are wired in. The CloudFormation template sets placeholder price IDs (price_starter_${Environment}, etc.) and never sets STRIPE_PRICE_NAT / STRIPE_PRICE_NAT_PRO, so the checkout Lambda falls back to non-existent defaults (price_nat_monthly, price_nat_pro_monthly) → 402 errors. There is also a pricing mismatch: the marketing site advertises the per-nation tiers $29 / $79, while older extension/code still references the legacy $49 / $149 / $399 per-seat tiers.

Acceptance Criteria

• Stripe products + recurring prices created for Nat ($29/mo) and Nat Pro ($79/mo) in the target Stripe account
• Real price IDs configured via CloudFormation parameters → STRIPE_PRICE_NAT / STRIPE_PRICE_NAT_PRO env vars on the checkout Lambda
• All legacy $49/$149/$399 tier references removed/updated so site, extension, and backend agree on the per-nation pricing
• A test checkout completes and the webhook creates/updates the nation record with the correct plan + query limit