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/recommendation/shuffle_test.go

@@ -112,7 +112,7 @@ func TestShuffle_OneCopyPerSong(t *testing.T) {
 	}
 }
 
-// A video rip held back from radio (M498 #5439) is never chosen, even to fill
+// A video rip held back from radio is never chosen, even to fill
 // the request.
 func TestShuffle_SkipsHeldBackRips(t *testing.T) {
 	suspect := "suspect"