feat: add independent dashboard health endpoint (#26)

tests/test_health.py

@@ -0,0 +1,6 @@
+from src.main import app, health
+
+
+def test_health_endpoint_reports_service_liveness_without_gitea_access():
+    assert any(getattr(route, "path", None) == "/healthz" for route in app.routes)
+    assert health() == {"status": "ok", "service": "stackchain-dashboard"}