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/requests.sql

@@ -111,7 +111,7 @@ SELECT * FROM requests
  LIMIT 1;
 
 -- name: RepointRequestsAlbumMbid :exec
--- A want moved from a release id to its group (#5241) takes the requests
+-- A want moved from a release id to its group takes the requests
 -- that name it along, so the reconciler finds it.
 UPDATE requests
    SET album_mbid = sqlc.arg(new_mbid),