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/models.py

@@ -141,6 +141,10 @@ class MelodyMainItem(BaseModel):
 
 class DeviceCreate(BaseModel):
     device_name: str = ""
+    # Console-only friendly name — set by staff, independent of the name the
+    # end user gave the device in the app. Never shown to app users, never
+    # synced from/to device_name. Falls back to device_name when empty.
+    console_name: str = ""
     device_photo: str = ""
     device_location: str = ""
     is_Online: bool = False
@@ -166,6 +170,7 @@ class DeviceCreate(BaseModel):
 
 class DeviceUpdate(BaseModel):
     device_name: Optional[str] = None
+    console_name: Optional[str] = None
     device_photo: Optional[str] = None
     device_location: Optional[str] = None
     is_Online: Optional[bool] = None