feat(settings): add model provider management with full CRUD support

Add a new settings page for managing API model providers (name, API URL,
API key, applicable agent types). Includes database migration, SeaORM
entity, backend CRUD commands/handlers, frontend settings UI with agent
type filter, add/edit/delete dialogs, and i18n support for all 10 locales.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/app/settings/model-providers/page.tsx

@@ -0,0 +1,5 @@
+import { ModelProviderSettings } from "@/components/settings/model-provider-settings"
+
+export default function SettingsModelProvidersPage() {
+  return <ModelProviderSettings />
+}