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/integration/agents/multi_agent_chat/test_kb_filesystem_cloud.py

@@ -23,7 +23,9 @@ from langgraph.checkpoint.memory import InMemorySaver
 from app.agents.multi_agent_chat.shared.middleware.filesystem import (
     build_filesystem_mw,
 )
-from app.agents.shared.filesystem_backends import build_backend_resolver
+from app.agents.multi_agent_chat.shared.middleware.filesystem.backends.resolver import (
+    build_backend_resolver,
+)
 from app.agents.shared.filesystem_selection import FilesystemMode, FilesystemSelection
 from tests.integration.harness import ScriptedTurn, build_scripted_harness
 

surfsense_backend/tests/integration/agents/multi_agent_chat/test_kb_filesystem_desktop.py

@@ -26,7 +26,9 @@ from langgraph.checkpoint.memory import InMemorySaver
 from app.agents.multi_agent_chat.shared.middleware.filesystem import (
     build_filesystem_mw,
 )
-from app.agents.shared.filesystem_backends import build_backend_resolver
+from app.agents.multi_agent_chat.shared.middleware.filesystem.backends.resolver import (
+    build_backend_resolver,
+)
 from app.agents.shared.filesystem_selection import (
     FilesystemMode,
     FilesystemSelection,