Allow provider_nodes to configure custom chat and models endpoint
paths via chatPath/modelsPath fields. This enables providers with
non-standard versioned APIs (e.g. /v4/chat/completions) to work
without embedding the version prefix in base_url.
- Add migration 003: chat_path and models_path columns
- Update Zod schemas (create, update, validate)
- Update CRUD in providers.ts (INSERT/UPDATE)
- Wire chatPath/modelsPath through API routes and providerSpecificData cascade
- Read chatPath in DefaultExecutor and BaseExecutor buildUrl()
- Use modelsPath in validate endpoint
- Add Advanced Settings UI section (collapsible) in create/edit modals
- Update base URL hint to reference Advanced Settings
- Add i18n keys across all 30 locales
- Add unit tests for buildUrl with custom paths
Backward compatible: NULL chatPath/modelsPath = default behavior.
