Replace the RSS-2.0-only SimpleXML parser with SimplePie so the Fetch RSS
node reads Atom 1.0 (YouTube, GitHub, The Verge…) and RSS 2.0 + namespace
extensions (dc:, content:, media:, yt:, itunes:). Each item exposes stable
cross-format aliases (title, link, date, content, author, …) plus every
namespaced field flattened for use as {{ fetched.* }}.
Add a feed-inspection endpoint that discovers a feed's real fields and feeds
them into the editor's expression autocomplete. Allow {{ }} expressions in the
feed URL via a ResolvableUrl rule. Parsing moves to a dedicated FeedParser
service; the fetch keeps the SSRF guard and gains XXE-safe parsing.
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