Add support for avoiding shifts on specific dates for workers

- Introduced AvoidShiftOnDates model to define constraints for workers.
- Enhanced Worker class to register avoid-shift rules with the rota builder.
- Implemented logic in RotaBuilder to enforce these constraints during shift assignment.
- Added tests to verify functionality for avoiding shifts on single dates and date ranges.

rota_generator/workers.py

@@ -111,6 +111,29 @@ class OutOfProgramme(BaseModel):
     reason: str = ""
 
 
+class AvoidShiftOnDates(BaseModel):
+    """Worker-level constraint: avoid listed shifts on given dates or date range."""
+    shifts: list[str]
+    start_date: Optional[datetime.date] = None
+    end_date: Optional[datetime.date] = None
+    dates: Optional[list[datetime.date]] = None
+
+    @field_validator("start_date", "end_date", mode="before")
+    def coerce_dates(cls, v):
+        # allow strings in common formats
+        if v is None:
+            return None
+        if isinstance(v, datetime.date):
+            return v
+        if isinstance(v, str):
+            for fmt in ("%d/%m/%Y", "%d/%m/%y", "%Y-%m-%d"):
+                try:
+                    return datetime.datetime.strptime(v, fmt).date()
+                except Exception:
+                    continue
+        raise ValueError(f"Cannot parse date: {v}")
+
+
 class Worker(BaseModel):
     name: str
     site: str
@@ -154,6 +177,7 @@ class Worker(BaseModel):
     shift_fte_overrides: dict[str, int] = {} # Need checks to ensure shifts exist
 
     weekend_shift_target_number: int = 0
+    avoid_shifts_on_dates: list[AvoidShiftOnDates] = []
 
 
 
@@ -395,6 +419,22 @@ class Worker(BaseModel):
                 style="red",
             )
 
+        # Register any per-worker avoid-shift-on-dates rules with the rota builder
+        try:
+            for av in getattr(self, "avoid_shifts_on_dates", []):
+                entry = {
+                    "names": [self.name],
+                    "shifts": av.shifts,
+                    "start_date": av.start_date,
+                    "end_date": av.end_date,
+                    "dates": av.dates,
+                }
+                # append to rota-level list; the model builder will enforce these
+                if hasattr(Rota.constraint_options_model, "avoid_shifts_by_worker_dates"):
+                    Rota.constraint_options_model.avoid_shifts_by_worker_dates.append(entry)
+        except Exception:
+            Rota.add_warning("Worker/avoid_shifts_on_dates", f"Failed to register avoid_shifts_on_dates for {self.name}")
+
     def __lt__(self, other) -> bool:
         return (self.site, self.grade, self.fte_adj, self.name) < (
             other.site,