fix(dashboard): stabilize four-column widget placement

src/govoplan_dashboard/backend/schemas.py

@@ -15,11 +15,29 @@ class DashboardWidgetPlacementPayload(BaseModel):
     instance_id: str = Field(min_length=1, max_length=120)
     widget_id: WidgetId
     size: Literal["small", "medium", "wide", "full"] = "medium"
+    column_start: int | None = Field(default=None, ge=1, le=4)
     configuration: dict[str, ConfigurationValue] = Field(
         default_factory=dict,
         max_length=40,
     )
 
+    @model_validator(mode="after")
+    def validate_column_span(self) -> "DashboardWidgetPlacementPayload":
+        if self.column_start is None:
+            return self
+        span = {
+            "small": 1,
+            "medium": 2,
+            "wide": 3,
+            "full": 4,
+        }[self.size]
+        if self.column_start + span - 1 > 4:
+            raise ValueError(
+                f"A {self.size} widget cannot start in column "
+                f"{self.column_start} of a four-column Dashboard."
+            )
+        return self
+
     @field_validator("configuration")
     @classmethod
     def validate_configuration(