fix(invitations): throttle unauthenticated invitation lookups and harden redirects

The invitation preview surfaces (/invitations/accept and /register?invitationToken=) are the
only unauthenticated reads of invitation data, and each render costs two database queries.
They are now rate limited before the lookup can touch the database: a generous per-IP bucket
that bounds enumeration across tokens, plus a tight per-IP+token bucket that stops repeated
probing of a single invitation. Tokens are hashed before they reach the rate_limits table.

A throttled lookup says so ("we couldn't check this invitation right now") instead of claiming
the invitation is invalid, and signed-in acceptance is not gated by it.

The callback sanitizer also checked only the origin, which is not enough: an attacker can
smuggle a host into the path of an otherwise same-origin URL — new URL('https://app/​/evil.com')
keeps our origin but yields a pathname of //evil.com, which navigation sinks resolve as
protocol-relative and follow off-site. Paths are now required to be rooted at a single slash,
and the login redirect re-checks at the sink.

getClientIp is split so server components that only have `await headers()` resolve the client
IP through the same trusted-proxy logic as route handlers.

app/(auth)/login/login-form.tsx

@@ -9,7 +9,11 @@ import { Card, CardContent, CardDescription, CardHeader, CardTitle } from '@/com
 import { Input } from '@/components/ui/input';
 import { Label } from '@/components/ui/label';
 import { signIn } from 'next-auth/react';
-import { getSafeCallbackUrl, isInvitationCallbackUrl } from '@/lib/safe-redirect';
+import {
+  getSafeCallbackUrl,
+  isInvitationCallbackUrl,
+  isSafeRelativePath,
+} from '@/lib/safe-redirect';
 
 /**
  * Sign-up link that carries the pending destination — and, when that destination is an
@@ -88,8 +92,10 @@ function LoginFormInner({ googleEnabled, githubEnabled }: LoginFormInnerProps) {
         return;
       }
 
+      // `result.url` is whatever next-auth resolved, so it is sanitized again here — and
+      // re-checked at the sink, because `router.push` happily leaves the origin.
       const destination = getSafeCallbackUrl(result?.url || callbackUrl);
-      router.push(destination);
+      router.push(isSafeRelativePath(destination) ? destination : '/dashboard');
       router.refresh();
     } catch {
       setError('Something went wrong. Please try again.');

app/(auth)/register/page.tsx

@@ -1,5 +1,6 @@
 import { isInviteCodeRequired } from '@/lib/feature-flags';
 import { getInvitationPreviewByToken } from '@/lib/invitations';
+import { isInvitationPreviewAllowed } from '@/lib/invitation-preview-limit';
 import RegisterPageClient from './register-page-client';
 
 interface RegisterPageProps {
@@ -11,7 +12,11 @@ export default async function RegisterPage({ searchParams }: RegisterPageProps)
   const githubEnabled = Boolean(process.env.GITHUB_CLIENT_ID && process.env.GITHUB_CLIENT_SECRET);
 
   const token = (await searchParams)?.invitationToken?.trim();
-  const preview = token ? await getInvitationPreviewByToken(token) : null;
+
+  // Unauthenticated invitation lookup — throttled per IP (and per token) before it can
+  // touch the database. When throttled we skip the lookup instead of guessing at a verdict.
+  const previewAllowed = token ? await isInvitationPreviewAllowed(token) : false;
+  const preview = token && previewAllowed ? await getInvitationPreviewByToken(token) : null;
   const invitation =
     preview && preview.status === 'PENDING' && !preview.isExpired
       ? {
@@ -29,6 +34,7 @@ export default async function RegisterPage({ searchParams }: RegisterPageProps)
       googleEnabled={googleEnabled}
       githubEnabled={githubEnabled}
       invitation={invitation}
+      invitationLookupThrottled={Boolean(token) && !previewAllowed}
     />
   );
 }

app/(auth)/register/register-page-client.tsx

@@ -24,6 +24,8 @@ interface RegisterPageClientProps {
   googleEnabled: boolean;
   githubEnabled: boolean;
   invitation?: RegisterInvitation | null;
+  /** Preview lookup was rate-limited, so `invitation` says nothing about its validity. */
+  invitationLookupThrottled?: boolean;
 }
 
 export default function RegisterPageClient({
@@ -31,6 +33,7 @@ export default function RegisterPageClient({
   googleEnabled,
   githubEnabled,
   invitation = null,
+  invitationLookupThrottled = false,
 }: RegisterPageClientProps) {
   const router = useRouter();
   const searchParams = useSearchParams();
@@ -245,6 +248,11 @@ export default function RegisterPageClient({
                     Create your account below — you'll be taken straight to it.
                   </p>
                 </div>
+              ) : isInvitationFlow && invitationLookupThrottled ? (
+                <div className="p-3 rounded-md bg-amber-500/10 text-sm">
+                  We couldn&apos;t check this invitation right now. Please wait a few minutes and
+                  open the link again.
+                </div>
               ) : isInvitationFlow ? (
                 <div className="p-3 rounded-md bg-amber-500/10 text-sm">
                   This invitation link is no longer valid. Ask whoever invited you for a new one.