Visione: quota non prodotta salvata come NaN (value NULL, fail) invece del 422; migrazione 022

Refs #118

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/backend/models/api/measurement.py

@@ -43,7 +43,7 @@ class MeasurementResponse(BaseModel):
     subtask_id: int
     version_id: int
     measured_by: int
-    value: float
+    value: Optional[float] = None  # #118: null = quota non prodotta dalla visione (NaN)
     pass_fail: str
     deviation: Optional[float] = None
     lot_number: Optional[str] = None
@@ -108,7 +108,7 @@ class CycleMeasurementResponse(BaseModel):
     """One quote already measured in the cycle under way."""
 
     subtask_id: int
-    value: float
+    value: Optional[float] = None  # #118: null = NaN
     pass_fail: str
     measured_at: UtcDateTime