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/search_test.go

@@ -28,7 +28,7 @@ const ramFeed = `<?xml version="1.0"?>
 </channel></rss>`
 
 // searchStack is a fake indexer feed and a fake FabledExchange download
-// client, configured the way the operator would in Integrations.
+// client, configured as an admin would in Admin › Acquisition.
 func searchStack(t *testing.T, ctx context.Context, store *acquire.Store) *atomic.Int32 {
 	t.Helper()
 	var added atomic.Int32