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/migrations/env.py

@@ -10,7 +10,7 @@ if config.config_file_name is not None:
     fileConfig(config.config_file_name)
 
 from app.database import Base
-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
 target_metadata = Base.metadata
 
 DATABASE_URL = os.getenv("DATABASE_URL", "postgresql+asyncpg://postgres:postgres@db:5432/taptrack_hub")