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>

backend/pyproject.toml

@@ -15,6 +15,9 @@ dependencies = [
     "python-dateutil>=2.9.0",
     "pydantic[email]>=2.10.0",
     "pydantic-settings>=2.6.0",
+    "apscheduler>=3.10.4",
+    "aiosmtplib>=3.0.1",
+    "zulip>=0.9.0",
 ]
 
 [project.optional-dependencies]