feat(measure): memorizza tempo di inserimento misura (input_duration_ms)

Colonna nullable su measurements + migration 003, campo opzionale negli
schemi API (create singola e batch), passthrough nel proxy Flask e timer
lato client in task_execute: parte all'attivazione del subtask, si
resetta su auto-advance e navigazione manuale.

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

src/backend/tests/test_measurements.py

@@ -55,6 +55,59 @@ class TestCreateMeasurement:
         assert data["pass_fail"] in ("pass", "warning", "fail")
         assert data["measured_by"] == measurement_tec_user.id
 
+    async def test_create_measurement_with_input_duration(
+        self,
+        client: AsyncClient,
+        measurement_tec_user: User,
+        db_session: AsyncSession,
+    ):
+        """input_duration_ms is stored and returned when provided."""
+        recipe = await create_test_recipe(
+            db_session, measurement_tec_user.id
+        )
+        subtask_id, version_id = await _get_subtask_and_version(
+            client, measurement_tec_user, recipe.id
+        )
+
+        resp = await client.post(
+            "/api/measurements/",
+            headers=auth_headers(measurement_tec_user),
+            json={
+                "subtask_id": subtask_id,
+                "version_id": version_id,
+                "value": 10.0,
+                "input_duration_ms": 4250,
+            },
+        )
+        assert resp.status_code == 200
+        assert resp.json()["input_duration_ms"] == 4250
+
+    async def test_create_measurement_without_input_duration(
+        self,
+        client: AsyncClient,
+        measurement_tec_user: User,
+        db_session: AsyncSession,
+    ):
+        """input_duration_ms defaults to null when omitted."""
+        recipe = await create_test_recipe(
+            db_session, measurement_tec_user.id
+        )
+        subtask_id, version_id = await _get_subtask_and_version(
+            client, measurement_tec_user, recipe.id
+        )
+
+        resp = await client.post(
+            "/api/measurements/",
+            headers=auth_headers(measurement_tec_user),
+            json={
+                "subtask_id": subtask_id,
+                "version_id": version_id,
+                "value": 10.0,
+            },
+        )
+        assert resp.status_code == 200
+        assert resp.json()["input_duration_ms"] is None
+
     async def test_measurement_requires_auth(self, client: AsyncClient):
         """Creating measurement without auth returns 401."""
         resp = await client.post(