fix(poll): reconcile concurrent respondent submissions

src/govoplan_poll/backend/participation_service.py

@@ -32,8 +32,10 @@ from govoplan_poll.backend.service import (
     _assert_governed_invitation_gateway_allowed,
     _assert_poll_accepts_responses,
     _existing_response,
+    _insert_or_reconcile_identified_response,
     _lock_poll_for_response,
     _now,
+    _update_existing_response,
     assert_no_sensitive_participation_metadata,
     get_poll_invitation_by_token,
     normalize_response_answers,
@@ -632,25 +634,32 @@ def _submit_locked_governed_response(
         trusted_metadata["comment"] = comment
     now = _now()
     if existing is not None:
-        if not poll.allow_response_update:
-            raise PollError("Response updates are not allowed for this poll")
-        existing.answers = normalized_answers
-        existing.respondent_label = payload.respondent_label
-        existing.submitted_at = now
-        existing.metadata_ = trusted_metadata
-        response = existing
+        response = _update_existing_response(
+            session,
+            poll=poll,
+            response=existing,
+            answers=normalized_answers,
+            respondent_label=payload.respondent_label,
+            submitted_at=now,
+            metadata=trusted_metadata,
+        )
     else:
-        response = PollResponse(
-            tenant_id=poll.tenant_id,
-            poll_id=poll.id,
+        response, _reconciled = _insert_or_reconcile_identified_response(
+            session,
+            poll=poll,
             respondent_id=respondent_id,
             respondent_label=payload.respondent_label,
             answers=normalized_answers,
             submitted_at=now,
-            metadata_=trusted_metadata,
+            metadata=trusted_metadata,
+            conflict_validator=lambda winner: _enforce_capacity(
+                session,
+                poll=poll,
+                existing=winner,
+                normalized_answers=normalized_answers,
+                limit=policy.max_participants_per_option,
+            ),
         )
-        session.add(response)
-    session.flush()
     if idempotency_key is not None:
         session.add(
             PollParticipationSubmission(