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_caps_test.go

@@ -50,9 +50,6 @@ func artistsOf(picks []Candidate) map[string]int {
 	return out
 }
 
-// THE BUG. Operator, 2026-09-10: started radio from a song and "literally
-// all of the songs in the playlist after that were from a single artist".
-//
 // One artist's tracks all outscore everything else, and there are more of
 // them than the radio has room for. Without a cap the output is entirely
 // that artist — nothing between the pool and the result bounded its share.