fix(AP1.7b): Audit user_id NULL bei Service-Token statt leerer UUID

Operational convert-backlog und patch-recurring nutzen ctx.user_id or None;
log_audit normalisiert leere Strings fuer audit_log.user_id.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/routers/operational.py

@@ -340,7 +340,7 @@ def operational_convert_backlog(
         result = backlog_service.convert_backlog_to_action(
             tenant_id=ctx.tenant_id,
             backlog_item_id=backlog_item_id,
-            user_id=ctx.user_id,
+            user_id=ctx.user_id or None,
             assigned_actor_ids=assigned,
             work_cycle_id=body.work_cycle_id,
             assign_active_sprint=body.assign_active_sprint,
@@ -375,7 +375,7 @@ def operational_patch_recurring(
         item = recurring_service.update_recurring_element(
             tenant_id=ctx.tenant_id,
             recurring_id=recurring_id,
-            user_id=ctx.user_id,
+            user_id=ctx.user_id or None,
             status=body.status,
         )
     except ValueError as exc:

backend/services/audit.py

@@ -23,7 +23,7 @@ def log_audit(
                 INSERT INTO audit_log (action, user_id, tenant_id, details)
                 VALUES (%s, %s, %s, %s::jsonb)
                 """,
-                (action, user_id, tenant_id, json.dumps(details or {})),
+                (action, user_id or None, tenant_id, json.dumps(details or {})),
             )
         conn.commit()
     finally: