refactor(routes): move cleanup domain into routes/cleanup/ subpackage (#5658)

Slice 2g of the route-domain reorganization (#4082/#4071). Moves
cleanup_routes.py into routes/cleanup/, leaving a backward-compat
sys.modules shim at the old path. Pure file reorganization, no behavior
change.

The shim uses sys.modules replacement so string-targeted
monkeypatch.setattr("routes.cleanup_routes.*", ...) in
test_cleanup_owner_scope.py reaches the canonical module.

Canonical module imports only from src/ and stdlib (zero internal
routes/ coupling). Zero source-introspection landmines.

Adds tests/test_cleanup_routes_shim.py to pin the sys.modules shim
contract. Verified: compileall clean; targeted tests pass.

routes/cleanup/__init__.py

@@ -0,0 +1,5 @@
+"""Cleanup route domain package (slice 2g, #4082/#4071).
+
+Contains cleanup_routes.py, migrated from the flat routes/ directory.
+Backward-compat shim at routes/cleanup_routes.py re-exports from here.
+"""

routes/cleanup/cleanup_routes.py

@@ -0,0 +1,60 @@
+# routes/cleanup_routes.py
+"""Routes for cleanup operations."""
+import logging
+from fastapi import APIRouter, HTTPException, Request
+from src.cleanup_service import get_cleanup_preview, cleanup_sessions
+from src.auth_helpers import get_current_user
+
+logger = logging.getLogger(__name__)
+
+def setup_cleanup_routes(session_manager):
+    """
+    Setup cleanup-related routes.
+
+    Args:
+        session_manager: SessionManager instance
+
+    Returns:
+        APIRouter instance with cleanup routes
+    """
+    router = APIRouter(prefix="/api/cleanup")
+
+    @router.get("/preview")
+    async def cleanup_preview(request: Request):
+        """
+        Preview what would be cleaned up without making any changes.
+
+        Returns:
+            JSON response with lists of sessions that would be archived/deleted and estimated space savings
+        """
+        user = get_current_user(request)
+        try:
+            preview = await get_cleanup_preview(owner=user)
+            return preview
+        except Exception as e:
+            logger.error(f"Cleanup preview failed: {e}")
+            raise HTTPException(500, "Cleanup preview generation failed")
+
+    @router.post("")
+    async def cleanup_endpoint(request: Request):
+        """
+        Perform cleanup operations:
+        1. Archive inactive sessions (not accessed for 7 days)
+        2. Delete old sessions (archived, not important, not accessed for 14+ days, with fewer than 10 messages)
+
+        Returns:
+            JSON response with counts of deleted and archived sessions, and space freed
+        """
+        user = get_current_user(request)
+        try:
+            archived_count, deleted_count, space_freed_mb = await cleanup_sessions(session_manager, owner=user)
+            return {
+                "archived_count": archived_count,
+                "deleted_count": deleted_count,
+                "space_freed_mb": round(space_freed_mb, 2)
+            }
+        except Exception as e:
+            logger.error(f"Cleanup failed: {e}")
+            raise HTTPException(500, "Cleanup operation failed")
+
+    return router

routes/cleanup_routes.py

@@ -1,60 +1,17 @@
-# routes/cleanup_routes.py
-"""Routes for cleanup operations."""
-import logging
-from fastapi import APIRouter, HTTPException, Request
-from src.cleanup_service import get_cleanup_preview, cleanup_sessions
-from src.auth_helpers import get_current_user
+"""Backward-compat shim — canonical location is routes/cleanup/cleanup_routes.py.
 
-logger = logging.getLogger(__name__)
+This module is replaced in ``sys.modules`` by the canonical module object so
+that ``import routes.cleanup_routes``, ``from routes.cleanup_routes import X``,
+``importlib.import_module("routes.cleanup_routes")``, and the string-targeted
+``monkeypatch.setattr("routes.cleanup_routes.get_cleanup_preview", ...)`` /
+``"routes.cleanup_routes.get_current_user"`` / ``"routes.cleanup_routes.
+cleanup_sessions"`` pattern used by test_cleanup_owner_scope.py all operate
+on the *same* object the application actually uses. Keeps existing import
+paths working after slice 2g (#4082/#4071).
+"""
 
-def setup_cleanup_routes(session_manager):
-    """
-    Setup cleanup-related routes.
+import sys as _sys
 
-    Args:
-        session_manager: SessionManager instance
+from routes.cleanup import cleanup_routes as _canonical  # noqa: F401
 
-    Returns:
-        APIRouter instance with cleanup routes
-    """
-    router = APIRouter(prefix="/api/cleanup")
-
-    @router.get("/preview")
-    async def cleanup_preview(request: Request):
-        """
-        Preview what would be cleaned up without making any changes.
-
-        Returns:
-            JSON response with lists of sessions that would be archived/deleted and estimated space savings
-        """
-        user = get_current_user(request)
-        try:
-            preview = await get_cleanup_preview(owner=user)
-            return preview
-        except Exception as e:
-            logger.error(f"Cleanup preview failed: {e}")
-            raise HTTPException(500, "Cleanup preview generation failed")
-
-    @router.post("")
-    async def cleanup_endpoint(request: Request):
-        """
-        Perform cleanup operations:
-        1. Archive inactive sessions (not accessed for 7 days)
-        2. Delete old sessions (archived, not important, not accessed for 14+ days, with fewer than 10 messages)
-
-        Returns:
-            JSON response with counts of deleted and archived sessions, and space freed
-        """
-        user = get_current_user(request)
-        try:
-            archived_count, deleted_count, space_freed_mb = await cleanup_sessions(session_manager, owner=user)
-            return {
-                "archived_count": archived_count,
-                "deleted_count": deleted_count,
-                "space_freed_mb": round(space_freed_mb, 2)
-            }
-        except Exception as e:
-            logger.error(f"Cleanup failed: {e}")
-            raise HTTPException(500, "Cleanup operation failed")
-
-    return router
+_sys.modules[__name__] = _canonical