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

backend/uv.lock

@@ -149,6 +149,7 @@ dependencies = [
     { name = "fastapi" },
     { name = "fastmcp" },
     { name = "greenlet" },
+    { name = "httpx2" },
     { name = "kurigram" },
     { name = "pydantic-settings" },
     { name = "rich" },
@@ -166,6 +167,7 @@ requires-dist = [
     { name = "fastapi", specifier = ">=0.115.0" },
     { name = "fastmcp", specifier = ">=3.3" },
     { name = "greenlet", specifier = ">=3.1.0" },
+    { name = "httpx2", specifier = ">=2.13.1" },
     { name = "kurigram", specifier = ">=2.2.23" },
     { name = "pydantic-settings", specifier = ">=2.7.0" },
     { name = "rich", specifier = ">=13.9.0" },