Introduce support for Traefik-native external middlewares referenced by resources.
- Database: add resource_external_middlewares table in migrations and ensure creation in post-migration updates.
- API: add handlers and routes to assign, remove and list external middlewares; include external_middlewares field in GetResources/GetResource responses (comma-separated name:priority:provider entries). Handlers validate resource status, use transactions, and log errors.
- Services: ConfigProxy now loads external middleware refs, merges them with internal middlewares sorted by priority when building resource config.
- UI: Resource detail component, API client, store and types updated to expose listing, assigning and removing external middlewares with UI controls and confirmation modal.
- Tests: add unit tests for assign/remove/list handlers and inclusion on GetResource.
This enables referencing middlewares defined outside MW-manager (e.g., Traefik dynamic config or plugins) and honors priority/provider metadata.
