API: use UTC timestamps in set attr value endpoint

The platform uses mostly UTC timestamps nowadays, and that also applies to
the data ingested. However, datapoints created through "set attribute value"
endpoint in the DP3 API still use timezone unaware timestamps.
This patch allows unified data processing in the applications.

Author: dbnk0

dp3/api/routers/entity.py

@@ -1,4 +1,4 @@
-from datetime import datetime
+from datetime import datetime, timezone
 from typing import Annotated, Any, Optional
 
 from fastapi import APIRouter, Depends, HTTPException, Request
@@ -354,7 +354,7 @@ async def set_eid_attr_value(
             id=eid,
             attr=attr,
             v=body.value,
-            t1=datetime.now(),
+            t1=datetime.now(timezone.utc),
             src=f"{request.client.host} via API",
         )
         dp3_dp = api_to_dp3_datapoint(dp.dict())