Podcast Index (api.podcastindex.org) ships as a disabled, key-less source
and is only consulted as a fallback when primary search results are fewer
than 3 — never on the hot path, never when disabled or credential-less.
A failed fallback leaves primary results intact.
Credentials are user-supplied: enabling the source pops a dialog that
asks for the free key+secret, prefilled masked (first 3 chars + "...")
when already stored; toggling off never clears them. Secrets prefer the
macOS keychain (security CLI, encrypted at rest) with a plaintext
config.json fallback when the keychain is unavailable; sources carry only
a hasCredentials/credentialStorage marker, and legacy plaintext keys in
existing configs are migrated on load.
Auth follows the documented scheme: X-Auth-Key, X-Auth-Date (epoch) and
Authorization = sha1(key + secret + date). Dead feeds are filtered, feed
URLs are used directly, and episode-scope search is a no-op (no endpoint).
