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).

routes/webhook/__init__.py

@@ -0,0 +1,5 @@
+"""Webhook route domain package (slice 2l, #4082/#4071).
+
+Contains webhook_routes.py, migrated from the flat routes/ directory.
+Backward-compat shim at routes/webhook_routes.py re-exports from here.
+"""