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/models/fusion_asset_depreciation_line.py

@@ -1,8 +1,4 @@
-"""Per-period depreciation board lines for an asset.
-
-Stub created with Task 8 (so fusion.asset One2many resolves). Fully
-elaborated in Task 9.
-"""
+"""Per-period depreciation board lines for an asset."""
 
 from odoo import fields, models
 
@@ -13,7 +9,34 @@ class FusionAssetDepreciationLine(models.Model):
     _order = "asset_id, scheduled_date"
 
     asset_id = fields.Many2one('fusion.asset', required=True, ondelete='cascade')
-    scheduled_date = fields.Date(required=True)
-    amount = fields.Monetary()
+    company_id = fields.Many2one(related='asset_id.company_id', store=True)
     currency_id = fields.Many2one(related='asset_id.currency_id', store=True)
-    is_posted = fields.Boolean(default=False)
+
+    period_index = fields.Integer(required=True)
+    scheduled_date = fields.Date(required=True)
+    amount = fields.Monetary(required=True)
+    accumulated = fields.Monetary()
+    book_value_at_end = fields.Monetary()
+
+    is_posted = fields.Boolean(default=False, copy=False)
+    posted_date = fields.Date(copy=False)
+    move_id = fields.Many2one(
+        'account.move', copy=False,
+        help="Journal entry created when this line was posted.",
+    )
+
+    def action_post(self):
+        """Mark this line as posted (without creating the journal entry yet —
+        engine method post_depreciation_entry handles the actual entry creation)."""
+        for line in self:
+            if line.is_posted:
+                continue
+            line.write({
+                'is_posted': True,
+                'posted_date': fields.Date.today(),
+            })
+
+    _unique_period_per_asset = models.Constraint(
+        'UNIQUE(asset_id, period_index)',
+        'A depreciation line for that period already exists.',
+    )