Make the clock zone a deployment setting (COFFER_TZ, default UTC)

coffer/product.py

@@ -6,6 +6,7 @@ from dataclasses import asdict
 from datetime import datetime
 from pathlib import Path
 from typing import Any
+import os
 from zoneinfo import ZoneInfo
 
 from coffer.db import CofferDB
@@ -30,7 +31,7 @@ from coffer.units import (
     to_subunits,
 )
 
-PT = ZoneInfo("Australia/Perth")
+PT = ZoneInfo(os.environ.get("COFFER_TZ", "UTC"))  # deployment sets its own zone
 
 
 def now_pt() -> str: