Round session cookie Max-Age without overflowing

Adding time.Second-1 to MEALQUEST_SESSION_MAX overflowed time.Duration
near its largest accepted value, giving a negative MaxAge that net/http
sends as Max-Age=0. Divide by time.Second and increment the quotient
when there is a remainder instead.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/auth/session.go

@@ -93,10 +93,21 @@ func (m *SessionManager) StartSession(w http.ResponseWriter, r *http.Request, cl
 	// The cookie outlives idle gaps so a returning device presents its ID;
 	// the server decides whether the session is still live. Max-Age is whole
 	// seconds, rounded up so the cookie never ends before the absolute limit.
-	http.SetCookie(w, sessionCookie(id, int((m.max+time.Second-1)/time.Second)))
+	http.SetCookie(w, sessionCookie(id, cookieMaxAge(m.max)))
 	return nil
 }
 
+// cookieMaxAge is lifetime in whole seconds, rounded up. It rounds the
+// quotient rather than adding to lifetime, which would overflow near the
+// largest time.Duration.
+func cookieMaxAge(lifetime time.Duration) int {
+	seconds := lifetime / time.Second
+	if lifetime%time.Second != 0 {
+		seconds++
+	}
+	return int(seconds)
+}
+
 // SessionFor returns the live session r carries and slides its last use to
 // now. It returns ErrNoSession when there is none; an expired session is
 // deleted.