Look up episodes by external identifier

Resolves one or more external episode identifiers or links to Particle episodes. Paste a Spotify, Apple Podcasts or YouTube episode link as it is and it is resolved by the platform it points to; links from different platforms may share a call, and platform is then optional. Bare identifiers need platform: Apple Podcasts episode IDs (the ?i= value in an Apple Podcasts URL), Spotify episode IDs and YouTube video IDs; the episode's RSS guid — the identifier the publisher's own feed carries, and the one most podcast tooling keys on — and a podcastindex episode ID; and hosting-platform IDs read from the episode's audio URL: megaphone, omny, acast and art19. Pass a comma-separated list (up to 100 per call). Guids and hosting-platform IDs are matched exactly as supplied and never read as links, since a guid is whatever string the feed carries and is often itself a URL. Because identifiers are comma-separated, a guid that itself contains a comma cannot be expressed here — about 1.2% of episodes, almost all of them SoundCloud-hosted, whose guids take the form tag:soundcloud,2010:tracks/123; look those episodes up by another identifier. Each result echoes the input identifier alongside the matched episode and its podcast; unresolved identifiers omit the episode field entirely so bulk callers can correlate inputs and outputs by key presence without doing a separate join. This is the episode-level counterpart to GET /v1/podcasts/lookup. Coverage is partial and differs by identifier, so a miss is not an error: Apple exposes only the most recent episodes of each show, so older back-catalogue episodes may not resolve; the first lookup of a Spotify episode can take a few seconds, and a call resolves at most 10 Spotify episodes not looked up before — retry the rest; YouTube resolves any video already discovered for an episode; guid is the broadest at roughly 89% of episodes; and a hosting-platform ID resolves only for episodes served by that host.