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/acoustid.sql

@@ -1,4 +1,4 @@
--- AcoustID recording-id lookup (M401). Migration 0069 says what each
+-- AcoustID recording-id lookup. Migration 0069 says what each
 -- lookup state means.
 
 -- name: ListTracksNeedingAcoustIDLookup :many
@@ -29,7 +29,7 @@ ON CONFLICT (track_id) DO UPDATE SET
     looked_up_at   = now();
 
 -- name: SetTrackMbidFromAcoustID :execrows
--- Never over a tag id (D4): the guard is here, not only in the worker, so a
+-- Never over a tag id: the guard is here, not only in the worker, so a
 -- tag the scan read between the worker's list and this write still wins.
 -- updated_at is left alone: it gates the scan's mtime skip, and moving it
 -- past a file's mtime would hide a change made just before the lookup.