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/frontend/flask_app/templates/maker/recipe_editor.html

@@ -556,10 +556,13 @@ function recipeEditor() {
         description: this.description.trim(),
       };
 
-      // Measurement interval
-      if (this.measurementIntervalMinutes) {
-        payload.measurement_interval_minutes = parseInt(this.measurementIntervalMinutes, 10);
-      }
+      // Measurement interval - always sent, explicit null when cleared
+      // (#37.3): omitting the key when the field was emptied used to leave
+      // the old value in place forever, because the backend could not tell
+      // "not sent" from "cleared".
+      payload.measurement_interval_minutes = this.measurementIntervalMinutes
+        ? parseInt(this.measurementIntervalMinutes, 10)
+        : null;
 
       // Rules of measurement. Sent every time, including when false: these are the
       // settings an operator gets told about, so turning one off has to travel.
@@ -567,10 +570,8 @@ function recipeEditor() {
       payload.requires_serial = !!this.requiresSerial;
       payload.allow_manual_input = !!this.allowManualInput;
 
-      // Include image_path for preview thumbnail
-      if (this.currentFilePath) {
-        payload.image_path = this.currentFilePath;
-      }
+      // image_path for preview thumbnail - same null-when-cleared rule (#37.3).
+      payload.image_path = this.currentFilePath || null;
 
       // Include change notes if editing
       if (!this.isNew && this.changeNotes.trim()) {