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/db/queries/system_playlists.sql

@@ -131,10 +131,10 @@ SELECT c.artist_id,
 --   tier 1  (only if tier 0 empty) all-time top non-skip plays
 --   tier 2  (only if tiers 0+1 empty) liked tracks
 -- Returns up to 5 ids; tie-break by track_id for determinism. The
--- Go-side picker (pickForYouSeedForDay) rotates one per day via
--- userIDHash. Widened from a hard 7-day window, which made For-You
--- disappear after a week of not listening and never recover on a
--- self-hosted library with sparse history.
+-- Go-side picker (pickDailySeeds) draws the day's seeds from these.
+-- Widened from a hard 7-day window, which made For-You disappear
+-- after a week of not listening and never recover on a self-hosted
+-- library with sparse history.
 WITH recent AS (
     SELECT t.id, COUNT(*) AS c, 0 AS tier
       FROM play_events pe