feat(backend): wire feed renderers into API

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>

apps/aelis-backend/src/tfl/renderer-provider.ts

@@ -0,0 +1,7 @@
+import type { FeedItemRenderer } from "@aelis/core"
+
+import { renderTflAlert } from "@aelis/source-tfl"
+
+export const TFL_SOURCE_ID = "aelis.tfl"
+
+export const tflRenderer: FeedItemRenderer = renderTflAlert as FeedItemRenderer