feat: add Quart app factory, config loader, and /api/health endpoint

Config reads from env vars (12-factor); .env.example documents defaults.
Health endpoint is liveness-only (no DB/Redis touch). Test added for CI.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/test_health.py

@@ -0,0 +1,20 @@
+"""Health endpoint smoke test."""
+
+import pytest
+
+from backend.app import create_app
+
+
+@pytest.fixture
+async def client():
+    app = create_app()
+    async with app.test_client() as c:
+        yield c
+
+
+@pytest.mark.asyncio
+async def test_health_returns_ok(client):
+    response = await client.get("/api/health")
+    assert response.status_code == 200
+    body = await response.get_json()
+    assert body == {"status": "ok"}