Harden admin tree state sync and add strict admin mode

[TimurKady]

Motivation

• Prevent client-side restoration of stale expanded-state after server-side tree changes by making the tree signature more robust and optionally disabling local restores in strict mode.
• Ensure DB-driven rendering avoids expensive per-row cached methods for critical visual cues (child presence, depth) to improve correctness and performance.
• Remove inconsistent local state after successful drag-and-drop moves so UI reflects the authoritative server snapshot after reload.

Description

• Added TREENODE_STRICT_ADMIN_MODE setting (treenode/settings.py) to enable a strict admin mode that disables client-side localStorage restoration of expanded state.
• Extended the changelist HTML to expose data-strict-admin-mode on the #changelist container and injected the flag from the admin view (treenode/admin/admin.py -> changelist_view).
• Expanded client getTreeSignature() in treenode/static/treenode/js/treenode_admin.js to include row order (index) and data-path (_path) and read data-strict-admin-mode; in strict mode the JS drops the stored state instead of restoring it.
• After a successful move/ AJAX call the client now removes the model-scoped expanded-state key from localStorage before calling location.reload() to avoid restoring stale state (treenode_admin.js).
• Emit data-path (from _path) and use _depth directly in server-rendered rows for both initial and AJAX row rendering by updating AdminMixin and AJAX row template (treenode/admin/mixin.py, treenode/templates/treenode/admin/treenode_ajax_rows.html).
• Reduce per-row cached-method usage by annotating changelist queryset with tn_children_count=Count('children') and using that annotated value (with fallback) for toggle/icon decisions and by using _depth for indentation (treenode/admin/admin.py, treenode/templatetags/treenode_admin.py).

Testing

• Ran python -m compileall treenode to validate Python files compile successfully (succeeded).
• Attempted python -m django test treenode.tests but it failed in this environment because DJANGO_SETTINGS_MODULE is not configured (test runner not available here).
• Attempted pytest -q but pytest-django could not auto-discover a Django project in this environment (no manage.py / configured project), so full test run was not possible here.

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Codex Task