feat(fusion_accounting_assets): depreciation board line model

- period_index, scheduled_date, amount, accumulated, book_value_at_end
- is_posted / posted_date / move_id (set when engine posts the entry)
- action_post() marks the line as posted (idempotent)
- UNIQUE(asset_id, period_index) constraint via models.Constraint
- 5 new tests (52 total)

Made-with: Cursor

fusion_accounting_assets/tests/__init__.py

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