feat: add email, scheduler, and Zulip integration services

- Add email service for sending meeting invites with ICS attachments
- Add scheduler for background calendar sync jobs
- Add Zulip service for meeting notifications
- Make ics_url optional for participants
- Add /api/schedule endpoint with 2-hour lead time validation
- Update frontend to support scheduling flow

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/pages/Index.tsx

@@ -118,7 +118,7 @@ const Index = () => {
       const created = await createParticipant({
         name: data.name,
         email: data.email,
-        ics_url: data.icsLink,
+        ics_url: data.icsLink || undefined,
       });
       setParticipants((prev) => [...prev, apiToParticipant(created)]);
       toast({