The provider verify call and the provider models-list call used hardcoded
timeouts (30s and 15s) that were too low for slow models. Introduce a
tenant-scoped, UI-editable setting `providers.request_timeout_sec`
(default 30) stored in system_configs, consumed by both handlers, and
overlaid from config.json like tts.timeout_ms. Adds a field to the System
Settings modal (all 4 locales) so operators can raise it without a
redeploy.
Co-authored-by: Bruno Clermont <bruno.clermont@gmail.com>
