feat(server): "You might like" album/artist Home rows (#790)

Surface in-library albums/artists the listener doesn't actively spin but
is predicted to enjoy, derived from the same similarity + like-weighted
candidate engine that powers For-You — rolled up from track scores to
album/artist granularity. Built in the daily 3am BuildSystemPlaylists
pass, atomic-replaced alongside the system playlists, and read back by
/api/home (+ /api/home/index).

Cold-start gate: skips generation entirely below 20 distinct unskipped
tracks AND 5 distinct artists, so a thin profile ships empty rows rather
than near-random tiles.

- migration 0034: you_might_like_albums / you_might_like_artists (id+rank,
  CASCADE, per-user rank index).
- playlists/you_might_like.go: cold-start gate + similarity roll-up
  (sum-of-top-3 aggregation, per-artist album cap, daily-rotating via the
  same userIDHash jitter as For-You) + atomic-replace persist in the tx.
- recommendation/home.go: two new HomePayload sections with read-time
  cross-section dedup vs Most Played / Rediscover / Last Played, trimmed
  to 10 each.
- api: you_might_like_albums / you_might_like_artists on /api/home and
  /api/home/index, reusing albumRefFrom / artistRefFromCovered.
- tests: pure roll-up/aggregation/cap unit tests + DB-backed gate,
  sufficiency, and atomic-replace tests (all green vs real Postgres).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/api/types.go

@@ -116,6 +116,11 @@ type HomePayload struct {
 	RediscoverArtists   []ArtistRef `json:"rediscover_artists"`
 	MostPlayedTracks    []TrackRef  `json:"most_played_tracks"`
 	LastPlayedArtists   []ArtistRef `json:"last_played_artists"`
+	// You-might-like: in-library albums/artists predicted from the user's
+	// listening that they don't actively spin. Built daily; gated on a
+	// minimum listening history so a thin profile gets empty rows.
+	YouMightLikeAlbums  []AlbumRef  `json:"you_might_like_albums"`
+	YouMightLikeArtists []ArtistRef `json:"you_might_like_artists"`
 }
 
 // HomeIndexPayload is the response body of GET /api/home/index — the
@@ -134,6 +139,8 @@ type HomeIndexPayload struct {
 	RediscoverArtists   []string `json:"rediscover_artists"`
 	MostPlayedTracks    []string `json:"most_played_tracks"`
 	LastPlayedArtists   []string `json:"last_played_artists"`
+	YouMightLikeAlbums  []string `json:"you_might_like_albums"`
+	YouMightLikeArtists []string `json:"you_might_like_artists"`
 }
 
 // HistoryEvent is one play in a user's listening history. Used by