Read youtube channels with the data api when the RSS feed fails

[pushkarsingh32]

YouTube's feeds/videos.xml has been failing since early October: for long stretches every channel and playlist feed returns 404, and at other times it flaps (in one run, 18 requests over 20 seconds came back as a mix of 200, 404 and 500). When it fails, Feed.Get returns failed to get <id>: 404 Not Found and the channel gets no new episodes.

This adds an optional youtube.api_key. When the feed request returns a non-200 status and a key is set, Feed.Get reads the same videos with the YouTube Data API's playlistItems.list (1 quota unit per call):

• channels: the uploads playlist (UC… -> UU…), playlists: the playlist itself
• results are mapped to the same Entry fields (video id, title, link, published/updated, description, thumbnail, author) and sorted newest first, so the rest of the service is unchanged
• private/deleted playlist entries (no publish date) are skipped
• the key goes in the X-Goog-Api-Key header, not the URL
• a warning is logged when the fallback is used

Without api_key nothing changes. APIBaseURL is only there so tests can point at an httptest server.

Tests:

• new table-driven TestFeed_GetFallsBackToAPI (channel, default type, playlist, no key keeps the feed error, API error), with a testdata/playlist_items.json fixture that includes a private video
• go test -race ./app/... passes
• live check against YouTube with the feed forced to 404: 3Blue1Brown channel returned 50 entries through the API (newest "The Phone Number puzzle", 2026-09-25) and a 16-video playlist returned 16

processor_test.go repeats the config's YouTube struct, so the new field is added there too. I didn't have golangci-lint locally; go vet and gofmt are clean for the changed files.

For setups that would rather not hold a Google Cloud key, getyoutubetranscript.com also offers a hosted endpoint for a channel's latest videos.