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/playlists/service_test_helpers_test.go

@@ -51,7 +51,7 @@ func newPool(t *testing.T) *pgxpool.Pool {
 }
 
 // seedUser inserts a `test-`-prefixed user (so dbtest.ResetDB can
-// clean it without touching the operator's admin row) and returns
+// clean it without touching the developer's admin row) and returns
 // the persisted user record.
 func seedUser(t *testing.T, pool *pgxpool.Pool, name string) dbq.User {
 	t.Helper()