feat(backend): webhooks deliver signed alerts from an outbox with retries and a test send

backend/pyproject.toml

@@ -20,6 +20,7 @@ dependencies = [
     "dishka>=1.4.0",
     "greenlet>=3.1.0",
     "fastmcp>=3.3",
+    "httpx2>=2.13.1",
 ]
 
 [build-system]