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/src/app/(app)/billing/page.tsx

@@ -7,7 +7,6 @@ import { MetalCard, TactileBadge } from '@d3ro/ui/components/ds'
 import { d3roFontMono } from '@d3ro/ui/theme'
 import { BillingCheckoutOptions } from '@/components/billing/billing-checkout-options'
 import { PaypleManageButton } from '@/components/billing/payple-manage-button'
-import { PortalButton } from '@/components/billing/portal-button'
 import { getSupabaseServerClient } from '@/lib/supabase-server'
 import {
   formatPlanCatalogPrice,
@@ -45,6 +44,7 @@ interface Plan {
 }
 
 const VALID_TIERS = new Set<SubscriptionTier>(['free', 'pro', 'pro_plus'])
+// 'stripe' 는 결제 경로가 제거된 뒤에도 과거 구독 행(provider='stripe')을 읽기 위해 남긴다.
 const VALID_PROVIDERS = new Set<BillingProvider>([
   'none',
   'stripe',
@@ -96,7 +96,7 @@ function providerLabel(provider: BillingProvider): string {
   const labels: Record<BillingProvider, string> = {
     none: '없음',
     payple: 'Payple',
-    stripe: 'Stripe',
+    stripe: 'Stripe (종료)',
     google_play: 'Google Play',
     app_store: 'App Store',
     admin: '관리자 부여'
@@ -164,8 +164,6 @@ export default async function BillingPage({ searchParams }: BillingPageProps): P
   const canPurchase = !isPaid && subscription.provider === 'none'
   const cancellationDate = formatDate(subscription.cancel_at)
   const periodEnd = formatDate(subscription.current_period_end)
-  const stripeCanceled = params['canceled'] === '1'
-  const stripeReturned = params['success'] === '1'
   const selectedTier = selectedTierFrom(params['tier'])
 
   return (
@@ -182,19 +180,6 @@ export default async function BillingPage({ searchParams }: BillingPageProps): P
         </Typography>
       </Box>
 
-      {stripeCanceled && (
-        <Alert severity="info" sx={{ mb: 2 }} data-testid="stripe-canceled-message">
-          Stripe Checkout을 취소했습니다. 결제나 구독 변경은 발생하지 않았습니다.
-        </Alert>
-      )}
-      {stripeReturned && (
-        <Alert severity={isPaid ? 'success' : 'warning'} sx={{ mb: 2 }} data-testid="stripe-return-message">
-          {isPaid
-            ? 'Stripe 결제가 확인되어 구독 정보가 갱신되었습니다.'
-            : 'Stripe 결제 결과를 확인 중입니다. 잠시 후 이 페이지를 새로고침해 주세요.'}
-        </Alert>
-      )}
-
       <MetalCard sx={{ p: { xs: 2.5, md: 3.5 }, mb: 4 }}>
         <Stack direction={{ xs: 'column', md: 'row' }} justifyContent="space-between" gap={3}>
           <Box>
@@ -257,7 +242,7 @@ export default async function BillingPage({ searchParams }: BillingPageProps): P
       </Box>
 
       <Typography sx={{ mt: 4, textAlign: 'center', color: 'var(--d3-text-label)', fontSize: 11 }}>
-        국내 카드는 Payple, 해외 카드는 Stripe가 처리합니다. 활성 구독이 있으면 같은 provider에서만 관리할 수 있습니다.
+        웹 결제는 Payple(원화 카드)이 처리합니다. 모바일 앱 구독은 Google Play에서 관리합니다.
       </Typography>
     </Box>
   )
@@ -350,7 +335,9 @@ function SubscriptionManagement({ subscription }: { subscription: BillingSubscri
     return <Typography sx={{ color: 'var(--d3-status-warning)', fontSize: 12 }}>자동 갱신이 해지되었습니다.</Typography>
   }
   if (subscription.provider === 'payple') return <PaypleManageButton />
-  if (subscription.provider === 'stripe') return <PortalButton />
+  if (subscription.provider === 'stripe') {
+    return <Alert severity="info">종료된 Stripe 결제로 만든 구독입니다. 변경·해지는 고객센터에 문의해 주세요.</Alert>
+  }
   if (subscription.provider === 'google_play') {
     return <Alert severity="info">Google Play 앱에서 구독을 관리해 주세요.</Alert>
   }

apps/web/src/app/(app)/dashboard/page.tsx

@@ -52,7 +52,7 @@ function tierLabel(tier: DashboardSnapshot['subscription']['tier']): string {
 function providerLabel(provider: DashboardSnapshot['subscription']['provider']): string {
   const labels: Record<DashboardSnapshot['subscription']['provider'], string> = {
     none: '미연결',
-    stripe: 'Stripe',
+    stripe: 'Stripe (종료)',
     payple: '페이플',
     google_play: 'Google Play',
     app_store: 'App Store',