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/alembic/versions/001_initial.py

@@ -22,7 +22,7 @@ def upgrade() -> None:
         sa.Column("id", sa.UUID(), nullable=False),
         sa.Column("name", sa.String(255), nullable=False),
         sa.Column("email", sa.String(255), nullable=False),
-        sa.Column("ics_url", sa.Text(), nullable=False),
+        sa.Column("ics_url", sa.Text(), nullable=True),
         sa.Column("created_at", sa.DateTime(), nullable=False),
         sa.Column("updated_at", sa.DateTime(), nullable=False),
         sa.PrimaryKeyConstraint("id"),