M8-T12: scope energy contracts

app/models/energy.py

@@ -163,8 +163,10 @@ class EnergyContract(Base):
 
     ``kind`` determines which price strategy is used (``manual`` for fixed
     dual-tariff rates entered by the user, ``tibber`` for dynamic API prices).
-    Only one contract may be ``active`` at a time; the service layer enforces
-    mutual exclusion.  Specific pricing values live in ``EnergyContractVersion``
+    A contract belongs to an energy ``scope`` (currently electricity; thermal
+    profiles are reserved for the next milestone). Only one contract may be
+    ``active`` per scope; the service layer enforces mutual exclusion. Specific
+    pricing values live in ``EnergyContractVersion``
     so that price changes can be tracked without modifying historical records.
     """
 
@@ -180,6 +182,12 @@ class EnergyContract(Base):
     # migration simple and the strategy registry extensible.
     kind: Mapped[str] = mapped_column(String(32), nullable=False)
 
+    # Billing domain. The service registry derives this from ``kind`` so API
+    # callers cannot move a pricing strategy into an incompatible domain.
+    scope: Mapped[str] = mapped_column(
+        String(32), nullable=False, default="electricity", index=True
+    )
+
     # Whether this is the currently active contract (at most one should be True).
     active: Mapped[bool] = mapped_column(Boolean, nullable=False, default=False)