feat(billing): polar-sync creates the Migrate-with-Studio bundle and yearly products

[ABB65]

What

pnpm polar:sync gets a third step that creates the four products Migrate with Studio sells through: per plan a bundle product (year 1 at 20% off) and a yearly list-price product. The Polar plugin already reads them via NUXT_POLAR_{STARTER,PRO}_{BUNDLE,YEARLY}_PRODUCT_ID.

• Prices come from shared/utils/migrate-bundle.ts (Starter $72 / $90, Pro $392 / $490), not a second copy.
• Products carry contentrain_variant; a product with no tag is monthly, so the existing Starter/Pro keep matching and are untouched.
• Create-if-missing, drift warns (exit 1) like the monthly products.
• Not in this PR: metered prices and meter credits on the yearly/bundle products. A yearly period grants credits per billing cycle, so the units are a pricing decision.

Evidence (sandbox)

• Dry run first: monthly products in sync, 4 products "would be created". Then --apply on sandbox only (NUXT_POLAR_SERVER=sandbox).
• A checkout opened the way createBundleCheckout does for 49000 (Migrate fee) + 7200 (Starter year 1): amount 56200, total 56200, bundle product, yearly interval. Paying it is not done here.
• Production: no product changes; env names and products ship with v0.4.8 on founder approval.

Reviewer: t6

[ABB65]

ONAY on 2742319 (local checks).

• Monthly products untouched: findExistingProduct now also requires (contentrain_variant ?? 'monthly') === 'monthly', so the two existing untagged products keep matching and the new tagged ones never match the monthly path; the new step 3 is create-if-missing only (an existing variant product only gets a price-drift warning, never a write).
• Env names printed (STARTER_BUNDLE, STARTER_YEARLY, PRO_BUNDLE, PRO_YEARLY → NUXT_POLAR_*_PRODUCT_ID) match nuxt.config.ts, .env.example and the keys the Polar plugin reads (starter#bundle, pro#bundle). Amounts come from migrate-bundle.ts (starter 7200/9000, pro 39200/49000 cents), matching the doc.
• Sandbox/prod: the target is NUXT_POLAR_SERVER (default sandbox, pnpm polar:sync pins .env); a production --apply is refused without --server=production; this PR adds no path around that. Nothing was run against Polar in review, so prod gets nothing without the founder's explicit go.
• pnpm lint 0 errors (7 existing warnings, none in polar-sync), pnpm typecheck clean, unit project 2037 passed; integration/nuxt projects not run: the diff is one script plus a doc that no integration or nuxt test imports.