FUE-T04: add Meter.uuid (stable HA identity anchor) + backfill migration

app/models/energy.py

@@ -11,6 +11,7 @@ Six tables:
 
 from __future__ import annotations
 
+import uuid as _uuid
 from datetime import datetime
 
 from sqlalchemy import Boolean, DateTime, Float, ForeignKey, Integer, String
@@ -20,6 +21,10 @@ from sqlalchemy.types import JSON
 from app.db import Base
 
 
+def _uuid4_str() -> str:
+    return str(_uuid.uuid4())
+
+
 class Meter(Base):
     """One physical electricity meter's installation epoch.
 
@@ -47,6 +52,11 @@ class Meter(Base):
 
     id: Mapped[int] = mapped_column(Integer, primary_key=True, autoincrement=True)
 
+    # Stable internal identity — used as HA Discovery unique_id anchor.
+    uuid: Mapped[str] = mapped_column(
+        String(36), unique=True, nullable=False, default=_uuid4_str
+    )
+
     # Human-readable label for this physical meter (e.g. address, serial, tariff zone).
     label: Mapped[str] = mapped_column(String(255), nullable=False)