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_aux_llm_owner_scope.py

@@ -16,7 +16,7 @@ def test_registered_manual_compaction_uses_session_owner_for_utility_endpoint():
 
 
 def test_task_name_generation_uses_owner_scoped_session_endpoint():
-    src = _src("routes/task_routes.py")
+    src = _src("routes/task/task_routes.py")
 
     assert "async def _generate_task_name(prompt: str, owner: Optional[str] = None)" in src
     assert "q = q.filter(DbSession.owner == owner)" in src