Add FeedRenderer and FeedRendererProvider to support server-side
rendering of feed items via ?render=json-render query param.
- FeedRenderer maps sourceId to FeedItemRenderer, renders matching
items and drops the rest
- FeedRendererProvider mirrors FeedSourceProvider pattern for
per-user renderer construction
- UserSession exposes renderer, handler converts JrxNode to Spec
- Returns 400 for unknown render format, 500 if renderer missing
Co-authored-by: Ona <no-reply@ona.com>
