feat: backend ingest API, Docker Compose, Ubuntu install guide

- FastAPI: POST /api/v1/events, GET /health, JSON Schema validation

- PostgreSQL models, Alembic migration, bootstrap API key

- deploy/docker-compose.yml, .env.example

- docs/install-ubuntu-24.04.md, updated work-plan (agents first)

backend/app/models/__init__.py

@@ -0,0 +1,5 @@
+from app.models.api_key import ApiKey
+from app.models.event import Event
+from app.models.host import Host
+
+__all__ = ["ApiKey", "Event", "Host"]