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/songs.sql

@@ -1,4 +1,4 @@
--- Song links (M498 #5438): copies of one song on different releases share a
+-- Song links: copies of one song on different releases share a
 -- song_key (migration 0076).
 
 -- name: LinkTracksAsSong :one
@@ -41,7 +41,7 @@ UPDATE tracks SET song_id = NULL
 
 -- name: ListTrackSongKeys :many
 -- What the mix writer needs to place a track: its song, and whether it is a
--- video rip held back from the mixes (#5439).
+-- video rip held back from the mixes.
 SELECT id, song_key, (source_verdict IS NOT DISTINCT FROM 'suspect')::boolean AS held_back
   FROM tracks WHERE id = ANY(sqlc.arg(ids)::uuid[]);