M8-R08: add atomic meter close and binding transfer

app/schemas/meter.py

@@ -146,3 +146,9 @@ class MeterPatchRequest(BaseModel):
             "Triggers billing recompute over the affected window."
         ),
     )
+
+
+class MeterCloseRequest(BaseModel):
+    """Close the active meter epoch at an exclusive end boundary."""
+
+    ended_at: datetime