refactor: use single date.today() call in get_default_date_range

Avoids potential midnight inconsistency between start and end of the
fallback period. Related to #50.

redmine_reporter/config.py

@@ -370,7 +370,8 @@ class Config:
     def get_default_date_range(cls) -> str:
         from_env = os.getenv("DEFAULT_FROM_DATE", "").strip()
         to_env = os.getenv("DEFAULT_TO_DATE", "").strip()
-        today_str = date.today().isoformat()
+        today = date.today()
+        today_str = today.isoformat()
 
         if from_env:
             return f"{from_env}--{to_env or today_str}"
@@ -392,7 +393,7 @@ class Config:
             default_to = cls._app.period_default_to or today_str
             return f"{cls._app.period_default_from}--{default_to}"
 
-        start = date.today().replace(day=1)
+        start = today.replace(day=1)
         return f"{start.isoformat()}--{today_str}"
 
     @classmethod