reject empty schedule_days with 422

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/server/models/config.py

@@ -112,6 +112,8 @@ class SchedulerConfig(BaseModel):
     @classmethod
     def validate_schedule_days(cls, v: List[str]) -> List[str]:
         """Validate each entry is a valid 3-letter lowercase day abbreviation."""
+        if not v:
+            raise ValueError("schedule_days cannot be empty")
         invalid = [d for d in v if d not in _VALID_DAYS]
         if invalid:
             raise ValueError(