docs: query comments say what each query selects and why, not how it came to be (#6277)

Rewrites the internal/db/queries comments to rule 32 and regenerates
dbq, whose doc comments and embedded SQL carry them. The bugs behind the
tiered mixes, the md5 order seed, the score bands and the round-robin
arms are restated as the constraint each one enforces; counts from one
library and stale cross-references are gone.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/db/queries/playlists.sql

@@ -55,7 +55,7 @@ RETURNING id, cover_path;
 -- (track was removed from the library). The denormalized snapshot fields
 -- on playlist_tracks remain authoritative for title/artist/album text.
 --
--- Deliberately NOT filtered on missing_since (#2527), unlike every browse /
+-- Deliberately NOT filtered on missing_since, unlike every browse /
 -- discover / mix query. A playlist entry is something the user put here on
 -- purpose, so a missing file stays in the list and renders as a dead row
 -- rather than silently vanishing; missing_since rides along so the service
@@ -76,7 +76,7 @@ ORDER BY pt.position;
 -- 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. pick_kind is NULL for manual
--- playlists; the For You builder stamps 'taste'/'fresh' (#1249).
+-- playlists; the For You builder stamps 'taste'/'fresh'.
 INSERT INTO playlist_tracks (playlist_id, position, track_id, title, artist_name, album_title, duration_sec, pick_kind)
 SELECT
     sqlc.arg(playlist_id)::uuid,
@@ -120,5 +120,5 @@ LIMIT $2;
 
 -- name: GetPlaylistsByIDs :many
 -- Batched lookup used by /api/library/sync to hydrate upsert payloads
--- (#357). Mirror of GetArtistsByIDs.
+--. Mirror of GetArtistsByIDs.
 SELECT * FROM playlists WHERE id = ANY($1::uuid[]);