feat: add Demo Requests module

- New demo_requests table (SQLAlchemy model + Alembic-ready)
- Public POST /api/demo-requests endpoint for the TapTrack website form
- Super-admin GET/PUT/DELETE endpoints to manage leads
- DemoRequestsPage.vue with stats row, filterable table, status updates, notes modal
- Sidebar nav item and route registered

backend/seed.py

@@ -8,7 +8,7 @@ from sqlalchemy.ext.asyncio import create_async_engine, async_sessionmaker
 from sqlalchemy import select
 from app.database import Base
 # Import all models so Base.metadata has the complete schema
-from app.models import user, school, license, sms, billing, ticket, audit, announcement, email_log, report  # noqa
+from app.models import user, school, license, sms, billing, ticket, audit, announcement, email_log, report, demo_request  # noqa
 from app.models.user import HubUser, UserRole
 from app.auth.password import hash_password