fix(proxy): allow unauthenticated access to /api/gdpr-export/[token]

The Clerk route matcher was redirecting the new GDPR export download
link to /sign-in, breaking the pawfeed.org token flow just shipped —
found while live-testing the link right after deploy.

src/proxy.ts

@@ -8,6 +8,7 @@ const isPublicRoute = createRouteMatcher([
   "/reset-password(.*)",
   "/verify-email(.*)",
   "/health-card/(.*)",
+  "/api/gdpr-export/(.*)",
   "/impressum",
   "/datenschutz",
   "/nutzungsbedingungen",