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/wanted/profiles.go

@@ -40,7 +40,7 @@ var (
 // which every profile passes on its primary type alone.
 //
 // A version its title names counts as a secondary type MusicBrainz left
-// off (#5573): "Common People (Recorded For BBC Radio 1)" is live whatever
+// off: "Common People (Recorded For BBC Radio 1)" is live whatever
 // its types say, and a version no type names, an acoustic one, no profile
 // passes.
 func Passes(p dbq.ReleaseTypeProfile, title, primary string, secondary []string) bool {