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/context_device_integration_test.go

@@ -33,7 +33,7 @@ func insertPlayEventDevice(
 
 // TestLoadContextAffinity_DeviceNarrowing seeds one artist played only on
 // mobile and another only on web (all in the current time cell), and verifies
-// the current-device parameter narrows the affinity cell (#1551): on 'mobile'
+// the current-device parameter narrows the affinity cell: on 'mobile'
 // the mobile artist out-scores the web artist and vice-versa, while the
 // device-agnostic (”) pass treats them equally.
 func TestLoadContextAffinity_DeviceNarrowing(t *testing.T) {