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>

bun.lock

@@ -25,6 +25,7 @@
         "@aelis/source-location": "workspace:*",
         "@aelis/source-tfl": "workspace:*",
         "@aelis/source-weatherkit": "workspace:*",
+        "@nym.sh/jrx": "^0.2.0",
         "@openrouter/sdk": "^0.9.11",
         "arktype": "^2.1.29",
         "better-auth": "^1",