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/events.sql.go

@@ -131,7 +131,7 @@ type GetSystemPickKindForTrackParams struct {
 // Looks a track up in the user's CURRENT snapshot of the given system
 // variant and returns its pick_kind. Used at play-ingestion time to
 // freeze provenance onto the play_event — snapshots rebuild daily, so
-// attribution can't be reconstructed at read time (#1249/#1270). No
+// attribution can't be reconstructed at read time. No
 // row = track not in today's snapshot (caller stores NULL); a row with
 // NULL pick_kind = the variant doesn't stamp (yet). songs_like_artist
 // is non-singleton (up to 3 mixes/user); a track in two of them takes
@@ -166,7 +166,7 @@ type InsertPlayEventParams struct {
 
 // pick_kind is non-NULL only for system-playlist plays whose track was
 // found (with a stamped kind) in the user's live snapshot for that
-// variant at ingestion time (#1249, generalized in #1270).
+// variant at ingestion time.
 func (q *Queries) InsertPlayEvent(ctx context.Context, arg InsertPlayEventParams) (PlayEvent, error) {
 	row := q.db.QueryRow(ctx, insertPlayEvent,
 		arg.UserID,