refactor(shared-rss): extract RSS parsing + Readability into one package

news-ingester and apps/api both shipped their own copy of rss-parser
+ jsdom + Readability glue. Single source now in packages/shared-rss.
Adds discoverFeeds (rel=alternate + common-paths probe) and validateFeed
which News Research will use. JSDOM virtualConsole is silenced once,
in the package, instead of in two parallel call sites.

- packages/shared-rss: parse, extract, discover, validate
- services/news-ingester: drop local parsers, depend on @mana/shared-rss
- apps/api: drop @mozilla/readability + jsdom direct deps, use shared

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/shared-rss/src/validate.ts

@@ -0,0 +1,24 @@
+import { parseFeedMeta } from './parse';
+import type { FeedValidation } from './types';
+
+const SAMPLE_LIMIT = 5;
+
+export async function validateFeed(url: string): Promise<FeedValidation> {
+	try {
+		const parsed = await parseFeedMeta(url);
+		return {
+			ok: parsed.items.length > 0,
+			itemCount: parsed.items.length,
+			title: parsed.title,
+			sample: parsed.items.slice(0, SAMPLE_LIMIT),
+		};
+	} catch (err) {
+		return {
+			ok: false,
+			itemCount: 0,
+			title: null,
+			sample: [],
+			error: err instanceof Error ? err.message : String(err),
+		};
+	}
+}