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/tests/test_recipes.py

@@ -250,6 +250,79 @@ class TestUpdateRecipe:
         assert version_numbers == [1, 2]
 
 
+class TestUpdateRecipeClearNullableFields:
+    """gitea#37.3: measurement_interval_minutes/image_path, once set, could
+    never be cleared again - the request set them and the update endpoint
+    could not tell "not sent" apart from "sent as null" (both landed on the
+    same getattr(...) is not None check in recipe_service._header_fields)."""
+
+    async def test_explicit_null_clears_measurement_interval(
+        self, client: AsyncClient, maker_user: User, db_session: AsyncSession,
+    ):
+        recipe = await create_test_recipe(db_session, maker_user.id)
+
+        set_resp = await client.put(
+            f"/api/recipes/{recipe.id}",
+            headers=auth_headers(maker_user),
+            json={"measurement_interval_minutes": 30},
+        )
+        assert set_resp.status_code == 200
+        assert set_resp.json()["measurement_interval_minutes"] == 30
+
+        clear_resp = await client.put(
+            f"/api/recipes/{recipe.id}",
+            headers=auth_headers(maker_user),
+            json={"measurement_interval_minutes": None},
+        )
+        assert clear_resp.status_code == 200
+        assert clear_resp.json()["measurement_interval_minutes"] is None
+
+    async def test_explicit_null_clears_image_path(
+        self, client: AsyncClient, maker_user: User, db_session: AsyncSession,
+    ):
+        recipe = await create_test_recipe(db_session, maker_user.id)
+
+        set_resp = await client.put(
+            f"/api/recipes/{recipe.id}",
+            headers=auth_headers(maker_user),
+            json={"image_path": "uploads/1/1/preview.png"},
+        )
+        assert set_resp.status_code == 200
+        assert set_resp.json()["image_path"] == "uploads/1/1/preview.png"
+
+        clear_resp = await client.put(
+            f"/api/recipes/{recipe.id}",
+            headers=auth_headers(maker_user),
+            json={"image_path": None},
+        )
+        assert clear_resp.status_code == 200
+        assert clear_resp.json()["image_path"] is None
+
+    async def test_field_not_sent_leaves_existing_value_untouched(
+        self, client: AsyncClient, maker_user: User, db_session: AsyncSession,
+    ):
+        """Regression guard for the other half of the distinction: a request
+        that never mentions the field must still leave it as it was."""
+        recipe = await create_test_recipe(db_session, maker_user.id)
+
+        set_resp = await client.put(
+            f"/api/recipes/{recipe.id}",
+            headers=auth_headers(maker_user),
+            json={"measurement_interval_minutes": 45},
+        )
+        assert set_resp.status_code == 200
+        assert set_resp.json()["measurement_interval_minutes"] == 45
+
+        unrelated_resp = await client.put(
+            f"/api/recipes/{recipe.id}",
+            headers=auth_headers(maker_user),
+            json={"name": "Renamed Recipe"},
+        )
+        assert unrelated_resp.status_code == 200
+        assert unrelated_resp.json()["measurement_interval_minutes"] == 45
+        assert unrelated_resp.json()["name"] == "Renamed Recipe"
+
+
 class TestDeleteRecipe:
     """DELETE /api/recipes/{recipe_id} tests."""