fix(devices): restore warranty/maintenance dates missing from timestamp field list

subscrStart, warrantyStart, and maintainedOn are written to Firestore as
Timestamps like the other date fields here, but were missing from
_TIMESTAMP_FIELD_NAMES, so they weren't being converted back to ISO
strings on read.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/devices/service.py

@@ -19,6 +19,7 @@ _TIMESTAMP_FIELD_NAMES = {
     "daySilenceFrom", "daySilenceTo",
     "nightSilenceFrom", "nightSilenceTo",
     "backlightTurnOnTime", "backlightTurnOffTime",
+    "subscrStart", "warrantyStart", "maintainedOn",
 }