feat(playlists): For You composition v2 — multi-seed blend + weighted fresh tail

Two approved composition changes (#1269), mechanism only — the
taste/fresh share stays data-decided (#1252) and pick_kind
attribution is unchanged.

Multi-seed blending: each day's build now seeds from up to 3 of the
user's top-5 tracks (pickDailySeeds, the generalized daily shuffle)
instead of one rotating anchor, so the mix spans neighborhoods within
a day and stops feeling bipolar as the rotation swings between
dissimilar seeds. Per-seed pools merge first-seen-deduped; the head
is filled best-first under 50/30/20 per-seed quotas (60/40 for two
seeds) so one neighborhood can't monopolize it, with thin-seed quota
spilling best-first.

Score-weighted fresh tail: the tail sample (rank 2*headN onward) was
uniform — the 380th-best candidate as likely as the 101st. It now
uses deterministic Efraimidis-Spirakis keys with weight halving every
50 ranks, so freshness keeps its "you'll probably enjoy this" half
while still rotating daily.

The retired single-seed picker's one other caller, You-might-like,
moves to pickDailySeeds(n=1) — a single neighborhood per day is right
for a short shelf, and the behavior note is inline.

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

internal/playlists/you_might_like.go

@@ -83,10 +83,14 @@ func buildYouMightLike(
 			"user_id", uuidStringPL(userID), "err", err)
 		return youMightLikeResult{built: false}
 	}
-	seed := pickForYouSeedForDay(seeds, userID, dateStr)
-	if !seed.Valid {
+	// One rotating seed is right here (unlike For-You's multi-seed
+	// blend, #1269): the row is a short shelf, not a mix, and a single
+	// neighborhood per day keeps it coherent.
+	daily := pickDailySeeds(seeds, userID, dateStr, 1)
+	if len(daily) == 0 {
 		return youMightLikeResult{built: true}
 	}
+	seed := daily[0]
 
 	zeroVec := recommendation.SessionVector{Seed: true}
 	// You-might-like surfaces in-library artists the user does NOT actively