refactor(agents): group filesystem backends under filesystem/backends/

The concrete filesystem backends are consumed only by the MAC filesystem
layer (tools, path-resolution middleware, the resolver, skills backend) and
tests -- no external app code. Group them next to the filesystem middleware
they serve:

- filesystem_backends.py            -> filesystem/backends/resolver.py
- middleware/kb_postgres_backend.py -> filesystem/backends/kb_postgres.py
- middleware/local_folder_backend.py -> filesystem/backends/local_folder.py
- middleware/multi_root_local_folder_backend.py -> .../multi_root_local_folder.py
- document_xml.py                   -> filesystem/backends/document_xml.py

Repoint all 21 importers. No behavior change; import-all + filesystem
backend/path-resolution/knowledge-search unit tests stay green (478).

surfsense_backend/tests/unit/middleware/test_filesystem_backends.py

@@ -2,16 +2,18 @@ from pathlib import Path
 
 import pytest
 
-from app.agents.shared.filesystem_backends import build_backend_resolver
+from app.agents.multi_agent_chat.shared.middleware.filesystem.backends.multi_root_local_folder import (
+    MultiRootLocalFolderBackend,
+)
+from app.agents.multi_agent_chat.shared.middleware.filesystem.backends.resolver import (
+    build_backend_resolver,
+)
 from app.agents.shared.filesystem_selection import (
     ClientPlatform,
     FilesystemMode,
     FilesystemSelection,
     LocalFilesystemMount,
 )
-from app.agents.shared.middleware.multi_root_local_folder_backend import (
-    MultiRootLocalFolderBackend,
-)
 
 pytestmark = pytest.mark.unit