feat(mixes): tiered rebuilds for New for you + First listens (rule #131)

Both mixes move from a single hard eligibility rule to the tiered
ladder, with their tier stamped onto playlist_tracks.pick_kind via the
#1270 provenance pipeline.

New for you (#1267) — consume on play, degrade by stepping back:
- "Consumed" = any track attempted >=30s; played albums leave the mix
  at the next build instead of crowding it until the calendar window
  expires.
- Tier 1: unconsumed albums added <30d by direct-affinity artists.
  Tier 2: unconsumed affinity albums from the wider 30-90d window —
  added while you weren't looking. Tier 3: any unconsumed album added
  <90d, newest first.

First listens (#1268) — track-level "attempted" threshold:
- A 2-second accidental brush no longer disqualifies a whole album;
  "attempted" is duration_played_ms >= 30000 per track.
- Tier 1: albums with zero attempted tracks. Tier 2: barely-attempted
  albums (<=25% of tracks reached 30s), minus the attempted tracks
  themselves. The artist-affinity ordering signal also moves to the
  >=30s definition so skip-only contact doesn't read as trust.

Producer plumbing: fetch adapters map the tier column onto pick kinds,
finishMix propagates PickKind into the persisted candidates, and
rotateForDay now rotates within contiguous same-pick-kind blocks so
daily rotation can't hoist tier-3 filler above tier-1's exact fits
(untiered pools are one block — original behavior).

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

internal/playlists/system_mixes.go

@@ -105,21 +105,39 @@ func produceDiscoveryMix(spec discoveryMixSpec) systemPlaylistProducer {
 	}
 }
 
-// rotateForDay rotates pool left by a daily-deterministic offset so
-// each day's downstream truncate-to-N surfaces a different slice of
-// the pool while contiguous-block ordering inside the slice is
-// preserved. Empty / single-element pools pass through unchanged.
+// rotateForDay rotates the pool left by a daily-deterministic offset
+// so each day's downstream truncate-to-N surfaces a different slice
+// while contiguous-block ordering inside the slice is preserved.
+//
+// Rotation happens WITHIN each contiguous same-pick-kind block
+// (#1267): tiered mixes arrive tier-ordered, and a whole-pool
+// rotation would hoist tier-3 filler above tier-1's exact fits.
+// Untiered pools are a single block, which reduces to the original
+// whole-pool rotation (same seed, same first draw). Empty /
+// single-element pools pass through unchanged.
 func rotateForDay(pool []discoverTrack, userID pgtype.UUID, dateStr string) []discoverTrack {
 	n := len(pool)
 	if n <= 1 {
 		return pool
 	}
 	rng := rand.New(rand.NewSource(int64(userIDHash(userID, dateStr))))
-	offset := rng.Intn(n)
-	rotated := make([]discoverTrack, 0, n)
-	rotated = append(rotated, pool[offset:]...)
-	rotated = append(rotated, pool[:offset]...)
-	return rotated
+	out := make([]discoverTrack, 0, n)
+	for start := 0; start < n; {
+		end := start + 1
+		for end < n && pool[end].PickKind == pool[start].PickKind {
+			end++
+		}
+		block := pool[start:end]
+		if len(block) > 1 {
+			offset := rng.Intn(len(block))
+			out = append(out, block[offset:]...)
+			out = append(out, block[:offset]...)
+		} else {
+			out = append(out, block...)
+		}
+		start = end
+	}
+	return out
 }
 
 // discoveryMixSpecs is the concrete spec list used by the registry in
@@ -173,7 +191,10 @@ var discoveryMixSpecs = []discoveryMixSpec{
 			}
 			out := make([]discoverTrack, len(rows))
 			for i, r := range rows {
-				out[i] = discoverTrack{ID: r.ID, AlbumID: r.AlbumID, ArtistID: r.ArtistID}
+				out[i] = discoverTrack{
+					ID: r.ID, AlbumID: r.AlbumID, ArtistID: r.ArtistID,
+					PickKind: pickKindForMixTier(r.Tier),
+				}
 			}
 			return out, nil
 		},
@@ -207,7 +228,10 @@ var discoveryMixSpecs = []discoveryMixSpec{
 			}
 			out := make([]discoverTrack, len(rows))
 			for i, r := range rows {
-				out[i] = discoverTrack{ID: r.ID, AlbumID: r.AlbumID, ArtistID: r.ArtistID}
+				out[i] = discoverTrack{
+					ID: r.ID, AlbumID: r.AlbumID, ArtistID: r.ArtistID,
+					PickKind: pickKindForMixTier(r.Tier),
+				}
 			}
 			return out, nil
 		},
@@ -244,7 +268,7 @@ func finishMix(rows []discoverTrack, diversify bool) []rankedCandidate {
 	}
 	tracks := make([]rankedCandidate, len(pool))
 	for i, t := range pool {
-		tracks[i] = rankedCandidate{TrackID: t.ID}
+		tracks[i] = rankedCandidate{TrackID: t.ID, PickKind: t.PickKind}
 	}
 	return tracks
 }