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/__manifest__.py

@@ -1,6 +1,6 @@
 {
     'name': 'Fusion Accounting Assets',
-    'version': '19.0.1.0.6',
+    'version': '19.0.1.0.7',
     'category': 'Accounting/Accounting',
     'summary': 'AI-augmented asset management with depreciation schedules.',
     'description': """