feat(recommendation): For You exploration attribution — taste vs fresh picks

Milestone #127 step 2 (#1249). For You deliberately blends two
populations — a head of top-scored taste picks and a tail sampled from
deeper ranking (the freshness injection) — but the metrics judged it as
one blob, so its skip rate couldn't distinguish "the taste engine is
missing" from "the freshness tax is too high". That number decides the
exploration share before we tune it.

- Migration 0038: nullable pick_kind ('taste'|'fresh') on both
  playlist_tracks (stamped at snapshot build) and play_events (frozen at
  play-ingestion — the snapshot rebuilds daily, so attribution cannot be
  reconstructed at read time).
- Builder: pickHeadAndTail marks head=taste / tail=fresh; the small-pool
  fallback is all taste (top-N-by-score IS the taste mechanism). Other
  variants persist NULL.
- Ingestion: for_you plays (live + offline replay) look the track up in
  the user's current snapshot; not found → unattributed, never guessed.
- Metrics: For You's row gains a breakdown (taste / fresh / earlier
  unattributed plays), parent row stays the sum; web card renders the
  sub-rows indented with the same baseline deltas + low-data dimming.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TsF3cNoKrqCYsU78cXC8U6

internal/playlists/system.go

@@ -127,12 +127,22 @@ func pickSeedArtistsForDay(pool []pgtype.UUID, userID pgtype.UUID, dateStr strin
 
 // rankedCandidate is a (track_id, score) pair used during in-memory
 // sorting before insert into playlist_tracks. T5 fills these from
-// recommendation.Candidate scores.
+// recommendation.Candidate scores. PickKind is set only by For-You's
+// head/tail composition (#1249); empty persists as NULL.
 type rankedCandidate struct {
-	TrackID pgtype.UUID
-	Score   float64
+	TrackID  pgtype.UUID
+	Score    float64
+	PickKind string
 }
 
+// For-You pick kinds, persisted on playlist_tracks.pick_kind so plays
+// can attribute skips to "the taste engine missing" vs "the freshness
+// tax" (#1249). Values mirror the CHECK in migration 0038.
+const (
+	pickKindTaste = "taste"
+	pickKindFresh = "fresh"
+)
+
 const systemMixLength = 25
 
 // systemMixWeights are the fixed scoring weights used by the cron worker.
@@ -632,14 +642,17 @@ func pickHeadAndTail(cands []recommendation.Candidate, userID pgtype.UUID, dateS
 	total := headN + tailN
 	if len(capped) <= total {
 		// Pool too small for a head/tail split — return up to total entries.
+		// All marked taste: top-N-by-score IS the taste mechanism; no
+		// exploration sampling happens on this path.
 		if len(capped) < total {
 			total = len(capped)
 		}
 		out := make([]rankedCandidate, total)
 		for i := 0; i < total; i++ {
 			out[i] = rankedCandidate{
-				TrackID: capped[i].Track.ID,
-				Score:   recommendation.Score(capped[i].Inputs, systemMixWeights, now, rng.Float64),
+				TrackID:  capped[i].Track.ID,
+				Score:    recommendation.Score(capped[i].Inputs, systemMixWeights, now, rng.Float64),
+				PickKind: pickKindTaste,
 			}
 		}
 		return out
@@ -667,15 +680,22 @@ func pickHeadAndTail(cands []recommendation.Candidate, userID pgtype.UUID, dateS
 
 	// Combine: head order preserved (score-sorted), tail in tieBreakHash
 	// order. "First similar, then surprise" reads naturally in playback.
+	// Head entries are the taste picks; tail entries are the freshness
+	// injection (#1249) — the split the metrics page attributes skips to.
 	combined := make([]recommendation.Candidate, 0, len(head)+len(tail))
 	combined = append(combined, head...)
 	combined = append(combined, tail...)
 
 	out := make([]rankedCandidate, len(combined))
 	for i, c := range combined {
+		kind := pickKindTaste
+		if i >= len(head) {
+			kind = pickKindFresh
+		}
 		out[i] = rankedCandidate{
-			TrackID: c.Track.ID,
-			Score:   recommendation.Score(c.Inputs, systemMixWeights, now, rng.Float64),
+			TrackID:  c.Track.ID,
+			Score:    recommendation.Score(c.Inputs, systemMixWeights, now, rng.Float64),
+			PickKind: kind,
 		}
 	}
 	return out
@@ -709,9 +729,15 @@ func insertSystemPlaylist(ctx context.Context, qtx *dbq.Queries, userID pgtype.U
 	}
 
 	for _, t := range tracks {
+		var pickKind *string
+		if t.PickKind != "" {
+			k := t.PickKind
+			pickKind = &k
+		}
 		if _, err := qtx.AppendPlaylistTrack(ctx, dbq.AppendPlaylistTrackParams{
 			PlaylistID: p.ID,
 			TrackID:    t.TrackID,
+			PickKind:   pickKind,
 		}); err != nil {
 			// Track may have been deleted between candidate-load and insert;
 			// skip silently rather than failing the whole build.