refactor(routes): move history domain into routes/history/ subpackage (#5090)

Slice 2d of the route-domain reorganization (#4082/#4071, per
specs/architecture-runtime-inventory.md §6.3). Moves history_routes.py into
routes/history/, leaving a backward-compat sys.modules shim at the old path.
Pure file reorganization, no behavior change.

The shim uses sys.modules replacement (same pattern as the merged gallery
#4903, research #4975, and memory #5007 slices) so that `import
routes.history_routes`, `from routes.history_routes import X`,
`importlib.import_module(...)`, and the `import ... as history_routes` +
`monkeypatch.setattr(history_routes, ...)` pattern used by
test_history_compact_tool_calls.py / test_fork_session_metadata.py all
operate on the same module object the application uses.

The canonical module does NOT depend on the shim — routes/history/
history_routes.py imports only from core/, src/, and routes.session_routes
(a sibling route module whose old import path stays valid via its own shim
when session is migrated later).

Three source-introspection test sites repointed to the new canonical path:
- test_history_db_fallback_hidden.py
- test_history_order_by_timestamp_regression.py
- test_model_helper_owner_scope.py

Adds tests/test_history_routes_shim.py to pin the sys.modules shim contract
(legacy and canonical paths resolve to the same module object; monkeypatch
via legacy alias reaches the canonical module).

Verified: compileall clean; full suite 4351 passed, 3 skipped.

app.py

@@ -645,7 +645,7 @@ from routes.research.research_routes import setup_research_routes
 app.include_router(setup_research_routes(research_handler, session_manager=session_manager))
 
 # History
-from routes.history_routes import setup_history_routes
+from routes.history.history_routes import setup_history_routes
 app.include_router(setup_history_routes(session_manager))
 
 # Search