refactor(routes): move task domain into routes/task/ subpackage

Slice 2p of the route-domain reorganization (#4082/#4071). Moves
task_routes.py (1181 lines) into routes/task/, leaving a backward-compat
sys.modules shim. Pure file reorganization, no behavior change.

The shim uses sys.modules replacement so the `import ... as task_routes` +
`monkeypatch.setattr(task_routes, "SessionLocal", ...)` /
`"get_current_user"` pattern and the `task_routes.__file__` reads in
test_auth_regressions.py all reach the canonical module.

Four source-introspection test sites repointed:
- test_aux_llm_owner_scope.py
- test_model_helper_owner_scope.py
- test_internal_api_base.py
- test_webhook_trigger_auth_exempt.py

Adds tests/test_task_routes_shim.py to pin the sys.modules shim contract.

Verified: compileall clean; full suite 5040 passed, 3 skipped.

tests/test_task_routes_shim.py

@@ -0,0 +1,20 @@
+"""Regression test for the task route shim (slice 2p, #4082/#4071).
+
+The backward-compat shim at ``routes/task_routes.py`` uses ``sys.modules``
+replacement so the legacy import path and the canonical ``routes.task.*``
+path resolve to the *same* module object. This is required because multiple
+tests do ``import routes.task_routes as task_routes`` followed by
+``monkeypatch.setattr(task_routes, "SessionLocal", ...)`` /
+``"get_current_user"``, and test_auth_regressions.py reads
+``task_routes.__file__`` for source introspection.
+"""
+
+import importlib
+
+import routes.task_routes as _shim_task  # noqa: F401
+
+
+def test_legacy_and_canonical_task_module_are_same_object():
+    legacy = importlib.import_module("routes.task_routes")
+    canonical = importlib.import_module("routes.task.task_routes")
+    assert legacy is canonical