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.go

@@ -169,6 +169,21 @@ func pickKindForSeedTier(tier int32) string {
 	}
 }
 
+// pickKindForMixTier maps a tiered mix query's 1-based tier column
+// (numbered to match rule #131's ladder directly) onto the pick-kind
+// vocabulary. Distinct from pickKindForSeedTier, whose 0-based tiers
+// count fallback steps of the seed pool rather than eligibility rungs.
+func pickKindForMixTier(tier int32) string {
+	switch tier {
+	case 1:
+		return pickKindTier1
+	case 2:
+		return pickKindTier2
+	default:
+		return pickKindTier3
+	}
+}
+
 const systemMixLength = 25
 
 // systemMixWeights are the fixed scoring weights used by the cron worker.