Fix: keep the session when the auth server is briefly unreachable

Any transient failure used to end the session: the token route deleted
the refresh cookies on every non-OK answer from the OAuth endpoint,
refreshAccessToken logged out on any non-OK status or network error,
and the startup restore evicted the account and deleted its session
cookie. A server restart, a proxy hiccup, a Wi-Fi switch or a laptop
waking before the network is back all kicked the user out despite
"stay signed in".

Failures are now classified. Only a definitive rejection (400/401/403
from the OAuth endpoint, 401 from the token route) tears the session
down and deletes cookies, exactly as before. Network errors and 5xx
keep the session: the token refresh re-arms itself and retries every
~30 seconds until the server is back, and the startup restore keeps
the account, marked unreachable - the same treatment the rate-limit
carve-out (#104) already applies.

Token validity stays entirely server-enforced: the first definitive
401 after an outage still logs out as before.

app/api/auth/token/route.ts

@@ -82,9 +82,16 @@ export async function PUT(request: NextRequest) {
     if (!tokenResponse.ok) {
       const errorText = await tokenResponse.text();
       logger.error('Token refresh failed', { status: tokenResponse.status, error: errorText });
-      cookieStore.delete(cookieName);
-      cookieStore.delete(refreshTokenServerCookieName(slot));
-      return NextResponse.json({ error: 'Refresh failed' }, { status: 401 });
+      // Drop the refresh token only when the server definitively rejected it
+      // (invalid/expired/revoked grant). A 5xx or 429 is an outage - keeping
+      // the cookie lets the session resume once the server is back.
+      const status = tokenResponse.status;
+      if (status === 400 || status === 401 || status === 403) {
+        cookieStore.delete(cookieName);
+        cookieStore.delete(refreshTokenServerCookieName(slot));
+        return NextResponse.json({ error: 'Refresh failed' }, { status: 401 });
+      }
+      return NextResponse.json({ error: 'Token endpoint unavailable' }, { status: 503 });
     }
 
     const tokens = await tokenResponse.json();