Adds a read-only GET /api/v1/settings/search-providers catalog (admin-gated, no secrets), grouped collapsible provider cards, and a schema-driven plugin config form. Breaks a SystemSettingService<->PluginManager circular dependency via parameter @Lazy (with a context smoke test), and fixes PluginManager.updateConfig to merge instead of overwrite so omitted/blank secret fields are preserved. Hardens plugin search-provider id validation (reject-not-trim, case-insensitive conflict) and insulates the provider bridge hot path from throwing plugin code.
