Explain autopilot picks in the UI (MAD-760)

[madeofpendletonwool]

People forgive picks they understand. Each DJ pick now carries a short reason and where the knowledge came from, and the app shows them. Phase 7, stage 4.

Server

• queue.AutopilotInfo gets Summary and Source. Autopilot writes them when it picks a song (autopilot/explain.go):

  Pick	Summary
  similar artist	Because Sam's Radiohead → Portishead (similar 0.82)
  two steps out	Further out from Sam's Radiohead → Portishead
  the room's artist	A top song by Radiohead, an artist the room loves (or "A deep cut by …")
  similar song	Like Jo's Creep
  bridge	Bridging Sam's Radiohead and Jo's Massive Attack
  throwback	A throwback to Blur, a favorite from past nights

  Picks with no member behind them say "the room's Radiohead". Source lists the music knowledge sources, e.g. "Last.fm, Deezer".

• The API's AutopilotPick has new reason and source fields. Songs a service recommended or that were picked at random don't have them, and still read "Like {seedTitle}".

• Summaries use members' display names as they were when the song was picked.

Web

• autopilotReason prefers the DJ's reason. The new autopilotSource returns "via Last.fm, Deezer".
• Up next: the autopilot row's byline shows the reason.
• Now playing sheet: the reason and its source sit under the badges.
• /tv: the reason and source show next to the Autopilot mark for the song playing, and a reason line is added on autopilot cards in up next.

Tests

• TestExplain covers each kind of pick. TestDJPicksFromTheGraph checks that the summary and source are stored.
• autopilot.test.ts covers the reason and source helpers.
• go test -race ./... passes, golangci-lint reports 0 issues, and the web suite (169 tests), tsc -b and oxlint are clean.

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