…60s limit (#5614)

* fix(backend): set hard time_limit on tasks that inherit the default 360s limit

schedule_update_search_data, schedule_ai_field_generation and run_async_job
declared no hard time_limit, so they inherited the global 360s default and were
killed mid-run on large workloads. Give each an explicit hard limit sized to its
real work, with the soft limit firing first.

Fixes BASEROW-SAAS-BACKEND-3Z

* Address PR review feedback (#5614)

- Clamp soft_time_limit to max(1, hard - 30) on the search and AI-field
  scheduling tasks so a misconfigured hard limit <= 30s can't break task
  registration at worker startup
- Remove the now-redundant inline comments explaining the explicit limits