feat(backend): wire feed renderers into API

[kennethnym]

What

Wire the FeedItemRenderer functions exported by source packages into the backend and expose them through the /api/feed endpoint via a ?render=json-render query parameter.

Changes

New types:

• FeedRenderer — holds a sourceId → FeedItemRenderer map. render(items) returns RenderedFeedItem[], silently dropping items with no matching renderer.
• FeedRendererProvider — interface with feedRendererForUser(userId): FeedRenderer, mirrors FeedSourceProvider.

Backend wiring:

• tfl/renderer-provider.ts and caldav/renderer-provider.ts export source IDs and renderer functions.
• UserSession accepts a FeedRenderer via options and exposes it as readonly renderer.
• UserSessionManager accepts a FeedRendererProvider and passes per-user FeedRenderer instances to sessions.
• server.ts aggregates renderer entries and wires the provider.

API behavior:

• GET /api/feed — unchanged.
• GET /api/feed?render=json-render — renders items via FeedRenderer, converts JrxNode → Spec (json-render format) in the handler, returns items with ui field.
• Unknown render value → 400.
• render=json-render with no renderer configured → 500.

Other:

• UserSession constructor now takes a single options object ({ sources, enhancer?, renderer? }).
• @nym.sh/jrx added as backend dependency.

[ona-patrol]

Both FeedItemRenderer functions and render() from @nym.sh/jrx can throw (e.g. malformed JRX node, duplicate keys, fragment at root). A single bad item would crash the entire request with an unhandled error.

Consider wrapping this in a try/catch to return a structured error, similar to how the enhancement pipeline handles failures gracefully:

Suggested change

	}))
	let renderedItems
	try {
	renderedItems = session.renderer.render(feed.items).map((item) => ({
	...item,
	ui: render(item.ui),
	}))
	} catch (err) {
	const message = err instanceof Error ? err.message : "Unknown rendering error"
	return c.json({ error: `Rendering failed: ${message}` }, 500)
	}

[ona-patrol]

Reviewed this PR and found 1 area that needs attention — see inline comment on the rendering error handling in http.ts.

Overall the implementation is well-structured: clean separation between FeedRenderer, FeedRendererProvider, and the wiring in server.ts. The UserSession constructor refactor to an options object is a good call. Test coverage is thorough — happy paths, filtering, edge cases, and error responses are all covered.