fix(recipe): measurement_interval_minutes e image_path si possono di nuovo azzerare (#37.3)

recipe_editor.html saveRecipe(): i due campi vengono sempre inviati nel
payload (valore o null esplicito), non più omessi quando svuotati.
recipe_service.py _header_fields(): per i due campi (unica eccezione
concordata al confine dei lotti, righe ~37-60) distingue "non inviato" da
"inviato null" con model_fields_set, invece di getattr(...) is not None —
altrimenti un null esplicito veniva scartato allo stesso modo di un campo
assente e il valore restava bloccato per sempre. Gli altri campi header
(name, description, requires_*, allow_manual_input) non toccati.
Test backend in test_recipes.py: TestUpdateRecipeClearNullableFields.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/backend/services/recipe_service.py

@@ -34,13 +34,34 @@ HEADER_FIELDS = (
 )
 
 
+# image_path and measurement_interval_minutes are nullable columns where the
+# caller may deliberately want to clear the value (send null), not merely
+# leave it untouched by not mentioning the field at all. A plain
+# "getattr(...) is not None" check cannot tell those two cases apart -
+# once set, neither field could ever be cleared again (#37.3). The other
+# header fields keep that simpler check: booleans are always sent by the
+# client (never null) and clearing name/description to null is not a case
+# this endpoint needs to support.
+_NULLABLE_HEADER_FIELDS = ("image_path", "measurement_interval_minutes")
+
+
 def _header_fields(data: RecipeUpdate) -> dict:
-    """The header columns the caller actually asked to change."""
-    return {
-        field: getattr(data, field)
-        for field in HEADER_FIELDS
-        if getattr(data, field) is not None
-    }
+    """The header columns the caller actually asked to change.
+
+    For image_path/measurement_interval_minutes, "asked to change" includes
+    explicitly setting it to null - told apart from "field absent from the
+    request" via model_fields_set, since RecipeUpdate defaults both to None.
+    """
+    result = {}
+    for field in HEADER_FIELDS:
+        if field in _NULLABLE_HEADER_FIELDS:
+            if field in data.model_fields_set:
+                result[field] = getattr(data, field)
+        else:
+            value = getattr(data, field)
+            if value is not None:
+                result[field] = value
+    return result
 
 
 async def _get_recipe_or_404(db: AsyncSession, recipe_id: int) -> Recipe: