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_internal_api_base.py

@@ -41,7 +41,7 @@ def test_no_hardcoded_loopback_left_in_call_sites():
         "src/tools/_common.py",
         "src/cookbook_serve_lifecycle.py",
         "src/builtin_actions.py",
-        "routes/task_routes.py",
+        "routes/task/task_routes.py",
     ):
         text = (root / rel).read_text(encoding="utf-8")
         # Allow it only inside comments; flag any code occurrence.