refactor(routes): move webhook domain into routes/webhook/ subpackage (#5781)

Slice 2l of the route-domain reorganization (#4082/#4071). Moves
webhook_routes.py into routes/webhook/, leaving a backward-compat
sys.modules shim. Pure file reorganization, no behavior change.
One source-introspection test repointed (test_api_chat_security.py).

tests/test_webhook_routes_shim.py

@@ -0,0 +1,11 @@
+"""Regression test for the webhook route shim (slice 2l, #4082/#4071)."""
+
+import importlib
+
+import routes.webhook_routes as _shim_webhook  # noqa: F401
+
+
+def test_legacy_and_canonical_webhook_module_are_same_object():
+    legacy = importlib.import_module("routes.webhook_routes")
+    canonical = importlib.import_module("routes.webhook.webhook_routes")
+    assert legacy is canonical