M8-T05: bind electricity costs to source bindings

app/schemas/energy.py

@@ -95,13 +95,18 @@ class CostPeriodSchema(BaseModel):
     export_revenue: float = Field(description="Revenue from electricity fed to grid (EUR).")
     net_cost: float = Field(description="import_cost − export_revenue (EUR).")
     currency: str = Field(description="ISO 4217 currency code.")
-    degraded: bool = Field(
-        description="True when the period was computed with incomplete data."
-    )
+    degraded: bool = Field(description="True when the period was computed with incomplete data.")
     contract_version_id: int | None = Field(
         default=None,
         description="FK to the contract version used for this billing period (null when degraded).",
     )
+    source_binding_id: int | None = Field(
+        default=None,
+        description=(
+            "FK to the source binding that supplied both cumulative endpoints "
+            "(null for legacy or degraded periods)."
+        ),
+    )
 
     model_config = {"from_attributes": True}