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/db/queries/events.sql

@@ -24,11 +24,27 @@ ORDER BY started_at DESC
 LIMIT 1;
 
 -- name: InsertPlayEvent :one
+-- pick_kind is non-NULL only for source='for_you' plays whose track was
+-- found in the user's live For You snapshot at ingestion time (#1249).
 INSERT INTO play_events (
-    user_id, track_id, session_id, started_at, client_id, source
-) VALUES ($1, $2, $3, $4, $5, $6)
+    user_id, track_id, session_id, started_at, client_id, source, pick_kind
+) VALUES ($1, $2, $3, $4, $5, $6, sqlc.narg(pick_kind)::text)
 RETURNING *;
 
+-- name: GetForYouPickKindForTrack :one
+-- Looks a track up in the user's CURRENT For You snapshot and returns
+-- its pick_kind ('taste'/'fresh'). Used at play-ingestion time to
+-- freeze exploration attribution onto the play_event — the snapshot is
+-- rebuilt daily, so attribution can't be reconstructed at read time
+-- (#1249). No row = track not in today's snapshot (caller stores NULL).
+SELECT pt.pick_kind
+FROM playlist_tracks pt
+JOIN playlists p ON p.id = pt.playlist_id
+WHERE p.user_id = $1
+  AND p.system_variant = 'for_you'
+  AND pt.track_id = $2
+LIMIT 1;
+
 -- name: UpdatePlayEventEnded :one
 -- Closes a play_event by id with the given ended_at, duration, and skip flag.
 -- completion_ratio is computed from duration_played_ms and the track duration