feat: add group management for workers and shifts, including validation for overlapping groups

rota_generator/workers.py

@@ -214,6 +214,7 @@ class Worker(BaseModel):
     shift_fte_overrides: dict[str, int] = {} # Need checks to ensure shifts exist
 
     exact_shifts: dict[str, int] = {} # Map shift_name to exact number of shifts
+    groups: set[str] = set() # Groups that the worker belongs to
 
     weekend_shift_target_number: float = 0.0
     avoid_shifts_on_dates: list[AvoidShiftOnDates] = []