refactor(routes): move gallery domain into routes/gallery subpackage (#4903)

Move the gallery route domain into routes/gallery/ while preserving backward-compatible legacy import shims.

- app imports the canonical gallery route module
- canonical gallery route code imports canonical gallery helpers
- legacy gallery route/helper paths remain compatibility aliases
- add shim regression coverage for module identity and monkeypatch behavior
- repoint gallery source-introspection tests to the canonical paths

No intended behavior change.

tests/test_gallery_routes_shim.py

@@ -0,0 +1,52 @@
+"""Regression test for the gallery route shim (slice 2a, #4082/#4071).
+
+The backward-compat shims at ``routes/gallery_routes.py`` and
+``routes/gallery_helpers.py`` use ``sys.modules`` replacement so the legacy
+import path and the canonical ``routes.gallery.*`` path resolve to the *same*
+module object. This test pins that contract: if the shim is ever changed to a
+plain ``from ... import *`` (or removed), these assertions catch it before the
+monkeypatch-based gallery tests silently start patching the wrong module.
+"""
+
+import importlib
+
+import routes.gallery_routes as _shim_routes  # noqa: F401
+import routes.gallery_helpers as _shim_helpers  # noqa: F401
+
+
+def test_legacy_and_canonical_route_module_are_same_object():
+    """``import routes.gallery_routes`` must alias the canonical module."""
+    legacy = importlib.import_module("routes.gallery_routes")
+    canonical = importlib.import_module("routes.gallery.gallery_routes")
+    assert legacy is canonical, (
+        "routes.gallery_routes shim must resolve to the canonical "
+        "routes.gallery.gallery_routes module object"
+    )
+
+
+def test_legacy_and_canonical_helpers_module_are_same_object():
+    """``import routes.gallery_helpers`` must alias the canonical module."""
+    legacy = importlib.import_module("routes.gallery_helpers")
+    canonical = importlib.import_module("routes.gallery.gallery_helpers")
+    assert legacy is canonical, (
+        "routes.gallery_helpers shim must resolve to the canonical "
+        "routes.gallery.gallery_helpers module object"
+    )
+
+
+def test_monkeypatch_via_legacy_path_affects_canonical(monkeypatch):
+    """Patching through the legacy path must reach the canonical module.
+
+    Several gallery tests do ``import routes.gallery_routes as gr`` followed by
+    ``monkeypatch.setattr(gr, "get_current_user", ...)``. For that to take
+    effect at runtime, the legacy module object and the canonical one must be
+    identical.
+    """
+    legacy = importlib.import_module("routes.gallery_routes")
+    canonical = importlib.import_module("routes.gallery.gallery_routes")
+
+    sentinel = object()
+    monkeypatch.setattr(legacy, "setup_gallery_routes", sentinel)
+    assert canonical.setup_gallery_routes is sentinel, (
+        "monkeypatch via legacy path did not reach the canonical module"
+    )