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/playlists.sql

@@ -68,8 +68,9 @@ ORDER BY pt.position;
 -- name: AppendPlaylistTrack :one
 -- Inserts at the next available position. Snapshot fields are copied
 -- from the tracks/albums/artists join at insert time. tracks.duration_ms
--- is converted to seconds for the snapshot.
-INSERT INTO playlist_tracks (playlist_id, position, track_id, title, artist_name, album_title, duration_sec)
+-- is converted to seconds for the snapshot. pick_kind is NULL for manual
+-- playlists; the For You builder stamps 'taste'/'fresh' (#1249).
+INSERT INTO playlist_tracks (playlist_id, position, track_id, title, artist_name, album_title, duration_sec, pick_kind)
 SELECT
     sqlc.arg(playlist_id)::uuid,
     COALESCE((SELECT MAX(position) + 1 FROM playlist_tracks WHERE playlist_id = sqlc.arg(playlist_id)::uuid), 0),
@@ -77,7 +78,8 @@ SELECT
     t.title,
     artists.name,
     albums.title,
-    (t.duration_ms / 1000)::integer
+    (t.duration_ms / 1000)::integer,
+    sqlc.narg(pick_kind)::text
 FROM tracks t
 JOIN albums  ON albums.id  = t.album_id
 JOIN artists ON artists.id = t.artist_id