chore: scaffold backend venv, health check, systemd unit

backend/app/main.py

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+from fastapi import FastAPI
+
+app = FastAPI(title="Quantumancy")
+
+
+@app.get("/healthz")
+async def healthz():
+    return {"status": "ok"}

backend/pytest.ini

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+[pytest]
+asyncio_mode = auto

backend/requirements.txt

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+fastapi==0.115.0
+uvicorn[standard]==0.32.0
+sqlalchemy[asyncio]==2.0.36
+asyncpg==0.30.0
+pydantic-settings==2.6.1
+argon2-cffi==23.1.0
+httpx==0.27.2
+numpy==2.1.3
+pytest==8.3.3
+pytest-asyncio==0.24.0

backend/tests/test_health.py

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+from fastapi.testclient import TestClient
+
+from app.main import app
+
+
+def test_healthz_returns_ok():
+    client = TestClient(app)
+    response = client.get("/healthz")
+    assert response.status_code == 200
+    assert response.json() == {"status": "ok"}