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/index.ts

@@ -0,0 +1,6 @@
+export type { NormalizedFeedItem, ExtractedArticle, DiscoveredFeed, FeedValidation } from './types';
+export { DEFAULT_USER_AGENT } from './types';
+export { parseFeedUrl, parseFeedXml, parseFeedMeta, type ParsedFeed } from './parse';
+export { extractFromUrl, extractFromHtml } from './extract';
+export { discoverFeeds, discoverFeedsFromSite, probeCommonPaths } from './discover';
+export { validateFeed } from './validate';