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/fingerprints.sql

@@ -1,6 +1,6 @@
 -- name: UpsertTrackFingerprint :exec
 -- Written whenever a track's fingerprint is derived: by the scan when a file is
--- new or its bytes changed, and by the backfill (#3908) for rows derived by an
+-- new or its bytes changed, and by the backfill for rows derived by an
 -- older method. Replaces the row wholesale — a fingerprint of the old bytes has
 -- no standing once the file has changed.
 INSERT INTO track_fingerprints (
@@ -23,7 +23,7 @@ ON CONFLICT (track_id) DO UPDATE SET
 DELETE FROM track_fingerprints WHERE track_id = $1;
 
 -- name: ListTracksNeedingFingerprint :many
--- The backfill's work queue (#3908): tracks with no fingerprint, or one derived
+-- The backfill's work queue: tracks with no fingerprint, or one derived
 -- by an older method. Keyset-paged on id so a pass visits each track at most
 -- once. That cursor is load-bearing: an inconclusive attempt writes no row, so
 -- without it a file that keeps timing out would be listed again straight away
@@ -34,7 +34,7 @@ SELECT t.id, t.file_path
   LEFT JOIN track_fingerprints f ON f.track_id = t.id
  WHERE t.missing_since IS NULL
    -- A row taken at another length is as stale as one from an older method:
-   -- chromaprints at two lengths cannot be compared (#3913).
+   -- chromaprints at two lengths cannot be compared.
    AND (f.track_id IS NULL
         OR f.fingerprint_version < sqlc.arg(current_version)
         OR f.chromaprint_length_sec <> sqlc.arg(chromaprint_length_sec))