refactor(routes): move cleanup domain into routes/cleanup/ subpackage (#5658)

Slice 2g of the route-domain reorganization (#4082/#4071). Moves
cleanup_routes.py into routes/cleanup/, leaving a backward-compat
sys.modules shim at the old path. Pure file reorganization, no behavior
change.

The shim uses sys.modules replacement so string-targeted
monkeypatch.setattr("routes.cleanup_routes.*", ...) in
test_cleanup_owner_scope.py reaches the canonical module.

Canonical module imports only from src/ and stdlib (zero internal
routes/ coupling). Zero source-introspection landmines.

Adds tests/test_cleanup_routes_shim.py to pin the sys.modules shim
contract. Verified: compileall clean; targeted tests pass.

tests/test_cleanup_routes_shim.py

@@ -0,0 +1,25 @@
+"""Regression test for the cleanup route shim (slice 2g, #4082/#4071).
+
+The backward-compat shim at ``routes/cleanup_routes.py`` uses ``sys.modules``
+replacement so the legacy import path and the canonical ``routes.cleanup.*``
+path resolve to the *same* module object. This is required because
+``test_cleanup_owner_scope.py`` uses string-targeted
+``monkeypatch.setattr("routes.cleanup_routes.get_cleanup_preview", ...)`` and
+``monkeypatch.delitem(sys.modules, "routes.cleanup_routes")`` + re-import —
+for those patches to take effect at runtime, the legacy module object and
+the canonical one must be identical.
+"""
+
+import importlib
+
+import routes.cleanup_routes as _shim_cleanup  # noqa: F401
+
+
+def test_legacy_and_canonical_cleanup_module_are_same_object():
+    """``import routes.cleanup_routes`` must alias the canonical module."""
+    legacy = importlib.import_module("routes.cleanup_routes")
+    canonical = importlib.import_module("routes.cleanup.cleanup_routes")
+    assert legacy is canonical, (
+        "routes.cleanup_routes shim must resolve to the canonical "
+        "routes.cleanup.cleanup_routes module object"
+    )