feat(studio): add self-hosted-only API 404 guard for the TanStack build

The Next pages-router build guards platform-only API routes via middleware
(proxy.ts, Next 16's renamed middleware convention). TanStack Start has no
middleware runtime, so that guard didn't run on the TanStack build at all.

Add an equivalent global request middleware on a TanStack start instance
(start.ts). On Vercel, /api/* and /_serverFn/* are rewritten to the
api/server.js function which runs createStartHandler, so requestMiddleware
executes server-side for every API request even though pages are a static
SPA shell. Extract the shared HOSTED_SUPPORTED_API_URLS allowlist into
lib/hosted-api-allowlist.ts so the Next and TanStack guards can't drift while
both frameworks run in parallel.

apps/studio/lib/hosted-api-allowlist.ts

@@ -0,0 +1,37 @@
+// [Joshen] Allowlist of API endpoints supported in hosted (platform) mode.
+// Every other /api/* route must 404 in platform mode. Shared by the Next
+// middleware (proxy.ts) and the TanStack request middleware (start.ts) so
+// the list can't drift between the two frameworks while both run in parallel.
+export const HOSTED_SUPPORTED_API_URLS = [
+  '/ai/sql/generate-v4',
+  '/ai/sql/policy',
+  '/ai/feedback/rate',
+  '/ai/code/complete',
+  '/ai/sql/cron-v2',
+  '/ai/sql/title-v2',
+  '/ai/sql/filter-v1',
+  '/ai/onboarding/design',
+  '/ai/feedback/classify',
+  '/ai/docs',
+  '/ai/sql/parse-client-code',
+  '/get-ip-address',
+  '/get-utc-time',
+  '/get-deployment-commit',
+  '/check-cname',
+  '/edge-functions/test',
+  '/edge-functions/body',
+  '/generate-attachment-url',
+  '/incident-status',
+  '/incident-banner',
+  '/status-override',
+  '/api/integrations/stripe-sync',
+  '/content/graphql',
+  '/parse-query',
+]
+
+// `pathname` must be basePath-relative — Next's `nextUrl.pathname` already is,
+// and the TanStack guard strips BASE_PATH before calling. Entries are path
+// suffixes, so `endsWith` stays correct regardless.
+export function isHostedSupportedApiPath(pathname: string): boolean {
+  return HOSTED_SUPPORTED_API_URLS.some((url) => pathname.endsWith(url))
+}

apps/studio/proxy.ts

@@ -1,44 +1,17 @@
 import type { NextRequest } from 'next/server'
 
 import { IS_PLATFORM } from '@/lib/constants'
+import { isHostedSupportedApiPath } from '@/lib/hosted-api-allowlist'
 
 export const config = {
   matcher: '/api/:function*',
 }
 
-// [Joshen] Return 404 for all next.js API endpoints EXCEPT the ones we use in hosted:
-const HOSTED_SUPPORTED_API_URLS = [
-  '/ai/sql/generate-v4',
-  '/ai/sql/policy',
-  '/ai/feedback/rate',
-  '/ai/code/complete',
-  '/ai/sql/cron-v2',
-  '/ai/sql/title-v2',
-  '/ai/sql/filter-v1',
-  '/ai/onboarding/design',
-  '/ai/feedback/classify',
-  '/ai/docs',
-  '/ai/sql/parse-client-code',
-  '/get-ip-address',
-  '/get-utc-time',
-  '/get-deployment-commit',
-  '/check-cname',
-  '/edge-functions/test',
-  '/edge-functions/body',
-  '/generate-attachment-url',
-  '/incident-status',
-  '/incident-banner',
-  '/status-override',
-  '/api/integrations/stripe-sync',
-  '/content/graphql',
-  '/parse-query',
-]
-
+// Return 404 for all next.js API endpoints EXCEPT the ones we use in hosted.
+// The allowlist is shared with the TanStack guard (start.ts) — see
+// lib/hosted-api-allowlist.ts.
 export function proxy(request: NextRequest) {
-  if (
-    IS_PLATFORM &&
-    !HOSTED_SUPPORTED_API_URLS.some((url) => request.nextUrl.pathname.endsWith(url))
-  ) {
+  if (IS_PLATFORM && !isHostedSupportedApiPath(request.nextUrl.pathname)) {
     return Response.json(
       { success: false, message: 'Endpoint not supported on hosted' },
       { status: 404 }

apps/studio/start.ts

@@ -0,0 +1,35 @@
+import { createMiddleware, createStart } from '@tanstack/react-start'
+
+import { BASE_PATH, IS_PLATFORM } from '@/lib/constants'
+import { isHostedSupportedApiPath } from '@/lib/hosted-api-allowlist'
+
+// Self-hosted-only API routes must 404 in platform (hosted) mode. Under the
+// Next pages router this lives in middleware (proxy.ts), but TanStack Start
+// has no middleware runtime, so the guard is migrated here as a global
+// request middleware sharing the same allowlist (lib/hosted-api-allowlist.ts).
+// On Vercel our `/api/*` (and `/_serverFn/*`) requests are rewritten to the
+// api/server.js function which runs the Start handler, so createStartHandler
+// runs this server-side for every API request — even though pages are served
+// as a static SPA shell. The guard therefore covers all API routes from a
+// single place.
+
+const platformApiGuard = createMiddleware({ type: 'request' }).server(({ request, next }) => {
+  const { pathname } = new URL(request.url)
+  // Path relative to the configured basePath — mirrors Next's basePath-
+  // relative middleware matcher.
+  const relativePath =
+    BASE_PATH && pathname.startsWith(BASE_PATH) ? pathname.slice(BASE_PATH.length) : pathname
+
+  if (IS_PLATFORM && relativePath.startsWith('/api/') && !isHostedSupportedApiPath(relativePath)) {
+    return Response.json(
+      { success: false, message: 'Endpoint not supported on hosted' },
+      { status: 404 }
+    )
+  }
+
+  return next()
+})
+
+export const startInstance = createStart(() => ({
+  requestMiddleware: [platformApiGuard],
+}))