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/components/ParticipantManager.tsx

@@ -25,16 +25,16 @@ export const ParticipantManager = ({
   const handleSubmit = (e: React.FormEvent) => {
     e.preventDefault();
 
-    if (!name.trim() || !email.trim() || !icsLink.trim()) {
+    if (!name.trim() || !email.trim()) {
       toast({
         title: "Missing fields",
-        description: "Please fill in all fields",
+        description: "Please fill in name and email",
         variant: "destructive",
       });
       return;
     }
 
-    onAddParticipant({ name: name.trim(), email: email.trim(), icsLink: icsLink.trim() });
+    onAddParticipant({ name: name.trim(), email: email.trim(), icsLink: icsLink.trim() || '' });
     setName('');
     setEmail('');
     setIcsLink('');
@@ -89,7 +89,7 @@ export const ParticipantManager = ({
             </div>
 
             <div className="space-y-2">
-              <Label htmlFor="icsLink">Calendar ICS Link</Label>
+              <Label htmlFor="icsLink">Calendar ICS Link (optional)</Label>
               <Input
                 id="icsLink"
                 placeholder="https://calendar.google.com/..."