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/dbq/scan_runs.sql.go

@@ -98,7 +98,7 @@ type InsertScanRunRow struct {
 	StartedAt pgtype.Timestamptz
 }
 
-// M7 #381: scan-run lifecycle queries.
+// Scan-run lifecycle queries.
 // Creates a new scan_runs row with started_at=now(). Stages are filled in
 // as each completes via UpdateScanRunStage; finished_at is set by FinishScanRun.
 func (q *Queries) InsertScanRun(ctx context.Context) (InsertScanRunRow, error) {
@@ -119,7 +119,7 @@ type UpdateScanRunArtistArtEnrichParams struct {
 	ArtistArtEnrich []byte
 }
 
-// M7 cover-sources: persists the artist-art enrich stage tally JSON.
+// Persists the artist-art enrich stage tally JSON.
 func (q *Queries) UpdateScanRunArtistArtEnrich(ctx context.Context, arg UpdateScanRunArtistArtEnrichParams) error {
 	_, err := q.db.Exec(ctx, updateScanRunArtistArtEnrich, arg.ID, arg.ArtistArtEnrich)
 	return err