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

tests/test_meters.py

@@ -381,6 +381,43 @@ class TestDeclareMeter:
         assert fetched.commodity == "gas"
         assert fetched.note == "Rotameter serial XYZ"
 
+    def test_declare_meter_generates_uuid(self, session: Session):
+        """declare_meter must auto-generate a non-empty uuid via ORM default."""
+        import re
+        UUID4_RE = re.compile(
+            r"^[0-9a-f]{8}-[0-9a-f]{4}-4[0-9a-f]{3}-[89ab][0-9a-f]{3}-[0-9a-f]{12}$",
+            re.IGNORECASE,
+        )
+
+        m = declare_meter(
+            session,
+            label="Meter with UUID",
+            started_at=_T0,
+            reason="initial",
+        )
+        session.commit()
+
+        fetched = session.get(Meter, m.id)
+        assert fetched is not None
+        assert fetched.uuid is not None, "uuid must not be None after declare_meter"
+        assert fetched.uuid != "", "uuid must not be empty"
+        assert UUID4_RE.match(fetched.uuid), (
+            f"uuid {fetched.uuid!r} does not look like a valid UUID v4"
+        )
+
+    def test_declare_meter_each_gets_distinct_uuid(self, session: Session):
+        """Each declared meter must receive a distinct UUID (not duplicated)."""
+        m1 = declare_meter(session, label="M1", started_at=_T0, reason="initial")
+        session.commit()
+
+        t1 = _T0 + timedelta(days=10)
+        m2 = declare_meter(session, label="M2", started_at=t1, reason="meter_swap")
+        session.commit()
+
+        assert m1.uuid != m2.uuid, (
+            f"Two declared meters must have distinct UUIDs; both got {m1.uuid!r}"
+        )
+
 
 # ---------------------------------------------------------------------------
 # 5. Different commodities are independent