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/admin_status.sql.go

@@ -109,7 +109,7 @@ type ListListeningNowRow struct {
 	AlbumTitle  string
 }
 
-// The admin Overview's status sections (M504): who is listening, what is
+// The admin Overview's status sections: who is listening, what is
 // downloading, what the library holds. Each is one cheap read.
 // Plays in progress: open play events still inside their track's length,
 // with a few minutes' grace for a pause. A client that never says a play