feat(fusion_accounting_assets): asset category template model

- defaults applied to new assets (method, useful_life, declining rate,
  salvage %, prorate convention)
- GL account hooks: asset_account_id, depreciation_account_id,
  expense_account_id (domain-filtered to relevant account types)
- computed asset_count for kanban / list views
- 3 new tests (55 total)

Made-with: Cursor

fusion_accounting_assets/tests/__init__.py

@@ -5,3 +5,4 @@ from . import test_asset_anomaly_detection
 from . import test_useful_life_predictor
 from . import test_fusion_asset
 from . import test_fusion_asset_depreciation_line
+from . import test_fusion_asset_category