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/library_test.go

@@ -70,7 +70,7 @@ func matchOf(t *testing.T, pool *pgxpool.Pool, album pgtype.UUID) (outcome, meth
 	return
 }
 
-// The library's albums go on the list as held, once (#5567). Tags that name
+// The library's albums go on the list as held, once. Tags that name
 // the group list it at once; a release id is named through MusicBrainz;
 // what nothing matches is recorded with why. Taken off the list, an album
 // stays off, and one added to the library afterwards is never listed.