refactor(billing): remove Stripe; payments are Payple (web) and Google Play (mobile)

Stripe is not used. Keeping its checkout, portal and webhook paths meant a
second payment provider, a second return-URL format and dead UI.

- Delete the stripe-checkout, stripe-portal and stripe-webhook functions and
  their config; billing-catalog serves Payple prices only, and the web parser
  rejects a catalog that still mixes in Stripe prices.
- Web: drop the Stripe checkout/portal buttons, provider toggle and return
  notices; billing shows Payple only. Past rows with provider='stripe' are
  still displayed ("Stripe (종료)") with a support contact instead of a portal.
- Desktop: delete the Stripe checkout modal, payment IPC channels, preload
  namespace and their types; "Remove ads with Pro" opens the web billing page
  via license.openBilling. Support/refund copy names Payple.
- billingUrl() loses the Stripe-only success/canceled result option; the
  Deno contract is regenerated.
- Migrations and the DB's accepted provider values are untouched (history).
- Docs and the backlog record the removal (MON-04, EXT-STRIPE-01, GAP-BILL-03).

Verified: typecheck (desktop/web/admin/api-client/mobile), contract:check,
deno check all functions, deno test 80/80, desktop 1478/1480 on the Electron
runtime (2 known environment failures), web and admin builds, release
metadata and mobile boundary self-tests, eslint on changed files.

apps/web/e2e/billing.spec.ts

@@ -138,10 +138,9 @@ test.describe('Billing provider and Payple DOM flow', () => {
     await expect(proPlan.getByTestId('payple-upgrade-pro')).toBeEnabled()
     await expect(page.getByTestId('billing-plan-pro_plus')).toBeVisible()
 
-    await proPlan.getByLabel('Stripe 해외 카드').click()
-    await expect(proPlan.getByTestId('stripe-upgrade-pro')).toBeVisible()
-    await proPlan.getByLabel('Payple 국내 카드').click()
+    await expect(proPlan.getByRole('button', { name: /Stripe/ })).toHaveCount(0)
 
+    // 결제 경로가 없어도 과거 provider='stripe' 구독 행은 계속 읽혀야 한다.
     const stripePeriodEnd = new Date(Date.now() + 30 * 24 * 60 * 60 * 1000).toISOString()
     const { error: stripeFixtureError } = await admin.from('subscriptions').update({
       tier: 'pro',
@@ -156,7 +155,7 @@ test.describe('Billing provider and Payple DOM flow', () => {
     if (stripeFixtureError) throw stripeFixtureError
     await page.reload()
     await expect(page.getByTestId('billing-current-provider')).toContainText('Stripe')
-    await expect(page.getByTestId('stripe-portal-open')).toBeVisible()
+    await expect(page.getByTestId('stripe-portal-open')).toHaveCount(0)
     await expect(page.getByTestId('billing-checkout-pro')).toHaveCount(0)
 
     const { error: freeFixtureError } = await admin.from('subscriptions').update({