feat(responses): support safe option editing

src/govoplan_poll/backend/service.py

@@ -732,6 +732,83 @@ def _lock_poll_for_response(session: Session, *, tenant_id: str, poll_id: str) -
     return poll
 
 
+def update_poll_option(
+    session: Session,
+    *,
+    tenant_id: str,
+    poll_id: str,
+    option_id: str,
+    label: str,
+    description: str | None,
+    value: dict[str, Any] | None,
+    metadata: dict[str, Any],
+) -> PollOption:
+    """Update one option and invalidate only answers bound to that option.
+
+    The Poll row is locked first, matching response submission.  The owning
+    API commits the option and response JSON updates in the same transaction.
+    """
+
+    poll = _lock_poll_for_response(
+        session,
+        tenant_id=tenant_id,
+        poll_id=poll_id,
+    )
+    if poll.status not in {"draft", "open"}:
+        raise PollError("Only draft or open poll options can be edited")
+    option = (
+        session.query(PollOption)
+        .filter(
+            PollOption.tenant_id == tenant_id,
+            PollOption.poll_id == poll.id,
+            PollOption.id == option_id,
+            PollOption.deleted_at.is_(None),
+        )
+        .populate_existing()
+        .with_for_update()
+        .one_or_none()
+    )
+    if option is None:
+        raise PollError("Unknown poll option")
+    normalized_value = dict(value) if value is not None else None
+    normalized_metadata = dict(metadata)
+    changed = (
+        option.label != label
+        or option.description != description
+        or option.value != normalized_value
+        or (option.metadata_ or {}) != normalized_metadata
+    )
+    if not changed:
+        return option
+
+    responses = (
+        session.query(PollResponse)
+        .filter(
+            PollResponse.tenant_id == tenant_id,
+            PollResponse.poll_id == poll.id,
+            PollResponse.deleted_at.is_(None),
+        )
+        .order_by(PollResponse.id.asc())
+        .populate_existing()
+        .with_for_update()
+        .all()
+    )
+    option.label = label
+    option.description = description
+    option.value = normalized_value
+    option.metadata_ = normalized_metadata
+    for response in responses:
+        retained_answers = [
+            answer
+            for answer in (response.answers or [])
+            if not isinstance(answer, dict) or answer.get("option_id") != option.id
+        ]
+        if retained_answers != (response.answers or []):
+            response.answers = retained_answers
+    session.flush()
+    return option
+
+
 def submit_poll_response(session: Session, *, tenant_id: str, poll_id: str, payload: PollSubmitResponseRequest) -> PollResponse:
     poll = _lock_poll_for_response(
         session,