docs: wanted, dbtest, recommendation and playlists comments say what the code does (#6277)

The same rewrite as d191a282, for four packages whose comments carried the
most history: tuning stories, quotes from the operator, single-install
measurements in library_scale.go, and 'used to' accounts of bugs. The
constraint each story protected stays as a present-tense reason (why tier 3
is reduced not removed, why the randomised arms take an order seed, why the
seed artist is eligible in Songs-like). Comments naming system_cron.go and
config.RecommendationConfig, neither of which holds what they claimed, now
name the scheduler and internal/recsettings.

Checked: non-comment tokens identical to HEAD; gofmt clean.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/playlists/song_copies_test.go

@@ -10,9 +10,9 @@ import (
 	"git.fabledsword.com/bvandeusen/minstrel/internal/playlists"
 )
 
-// A mix holds a song once (M498 #5438) and never a held-back video rip
-// (#5439): of a single and its album cut, the better-ranked copy stays; a rip
-// is left out and the next copy of its song takes the place.
+// A mix holds a song once and never a held-back video rip: of a single and its
+// album cut, the better-ranked copy stays; a rip is left out and the next copy
+// of its song takes the place.
 func TestOneCopyPerSong_Integration(t *testing.T) {
 	pool := newPool(t)
 	ctx := context.Background()