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/scan_runs.sql

@@ -1,4 +1,4 @@
--- M7 #381: scan-run lifecycle queries.
+-- Scan-run lifecycle queries.
 
 -- name: InsertScanRun :one
 -- Creates a new scan_runs row with started_at=now(). Stages are filled in
@@ -33,7 +33,7 @@ SELECT id, started_at, finished_at, library, mbid_backfill, cover_enrich, artist
  LIMIT 1;
 
 -- name: UpdateScanRunArtistArtEnrich :exec
--- M7 cover-sources: persists the artist-art enrich stage tally JSON.
+-- Persists the artist-art enrich stage tally JSON.
 UPDATE scan_runs
    SET artist_art_enrich = $2
  WHERE id = $1;