feat(recommendation): Songs-like can include the seed artist's own music

Operator, 2026-09-10: "it should also be able to include music from the same
artist." Completes #3881 — the weights and pool landed in f367eeaa; this is
the eligibility half.

produceSeedMixes filtered the seed artist out entirely:

    // "Songs like X" excludes X's own songs.
    if !pgtypeUUIDEqual(c.Track.ArtistID, artistID) { ... }

That reads as obviously right and is not. The seed is a TRACK — the artist's
top-played one — and the tracks most likely to sound like it are usually the
rest of that artist's catalogue. The filter threw away the seed's nearest
neighbours, then reached FURTHER OUT to replace them. On the one surface
whose job is staying in a neighbourhood, that is backwards, and it worked
against the coherence tuning rather than with it.

Domination is bounded by the cap instead of by exclusion, which is the
distinction that makes this safe rather than a new problem:
capCandidatesByAlbumAndArtist already allows at most 3 tracks per artist in
a 25-track mix, so the seed artist gets 12% at most — a presence, not a
takeover. Without that bound this would just be the radio failure (#3882)
arriving on a different surface. The seed track itself still cannot appear;
it is passed to LoadCandidatesFromSimilarity as an exclusion.

Guarded end-to-end rather than by reading the source, for two reasons: the
check has to survive the filter returning in a different shape, and an
absence check would now match the comment that explains why the filter is
gone — rule 167's prose trap exactly. The test asserts both directions, that
at least one mix contains its seed artist and that none exceeds the cap.

Its falsification is by construction rather than by execution: under the
previous code every mix's own-artist count was necessarily zero, so the
assertion could not have passed. Running it needs Postgres, which is the
integration lane's job.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SQ31KQpYbStyK5y58UmPLH

internal/playlists/system_test.go

@@ -287,6 +287,79 @@ func TestBuildSystemPlaylists_Concurrency(t *testing.T) {
 	}
 }
 
+// The seed artist's own tracks are ELIGIBLE for its "Songs like" mix, and are
+// bounded by the diversity cap rather than excluded outright (#3881).
+//
+// produceSeedMixes used to filter them out — "Songs like X excludes X's own
+// songs" — which threw away the nearest neighbours of the seed TRACK and then
+// reached further out to replace them. Operator, 2026-09-10: "it should also
+// be able to include music from the same artist."
+//
+// Asserted end-to-end rather than by reading the source, because the guard has
+// to survive the filter coming back in a different shape — and because an
+// absence check would now match the comment explaining why the filter is gone.
+func TestBuildSystemPlaylists_SongsLikeIncludesItsSeedArtist(t *testing.T) {
+	pool := newPool(t)
+	logger := discardLogger()
+	u, _ := seedActiveLibrary(t, pool, "seedartist", 4, 5)
+	ctx := context.Background()
+	now := time.Date(2026, 5, 4, 12, 0, 0, 0, time.UTC)
+
+	if err := playlists.BuildSystemPlaylists(ctx, pool, logger, u.ID, now, t.TempDir()); err != nil {
+		t.Fatalf("build: %v", err)
+	}
+
+	rows, err := pool.Query(ctx, `
+		SELECT count(*) FILTER (WHERE t.artist_id = p.seed_artist_id) AS own,
+		       count(*)                                              AS total
+		  FROM playlists p
+		  JOIN playlist_tracks pt ON pt.playlist_id = p.id
+		  JOIN tracks t          ON t.id = pt.track_id
+		 WHERE p.user_id = $1 AND p.system_variant = 'songs_like_artist'
+		 GROUP BY p.id
+	`, u.ID)
+	if err != nil {
+		t.Fatalf("query: %v", err)
+	}
+	defer rows.Close()
+
+	// discoverMaxTracksPerArtist, which this package_test cannot reference.
+	// Duplicated deliberately: if the cap moves, this failing is the point.
+	const maxPerArtist = 3
+
+	mixes, withOwn := 0, 0
+	for rows.Next() {
+		var own, total int
+		if err := rows.Scan(&own, &total); err != nil {
+			t.Fatalf("scan: %v", err)
+		}
+		mixes++
+		if own > 0 {
+			withOwn++
+		}
+		// The bound is what makes inclusion safe. Without it, "include the
+		// seed artist" becomes "the mix is mostly the seed artist", which is
+		// the radio failure (#3882) arriving on a different surface.
+		if own > maxPerArtist {
+			t.Errorf("a songs_like mix carries %d tracks by its own seed artist "+
+				"out of %d; the per-artist cap (%d) is not being applied",
+				own, total, maxPerArtist)
+		}
+	}
+	if err := rows.Err(); err != nil {
+		t.Fatalf("rows: %v", err)
+	}
+
+	if mixes == 0 {
+		t.Fatal("no songs_like_artist mixes were built, so this test asserts nothing")
+	}
+	if withOwn == 0 {
+		t.Errorf("none of the %d songs_like mixes contains a single track by its own "+
+			"seed artist — the tracks most likely to sound like the seed are being "+
+			"excluded from the surface whose job is sounding like the seed", mixes)
+	}
+}
+
 func TestBuildSystemPlaylists_DailyNonceDeterminism(t *testing.T) {
 	pool := newPool(t)
 	logger := discardLogger()