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/scheduler.go

@@ -1,5 +1,4 @@
-// Per-user system-playlist scheduler (#392 Half B). Replaces the
-// 24h-anchored cron loop in system_cron.go (deleted in the same arc).
+// Per-user system-playlist scheduler.
 //
 // Each active user gets one gocron daily job at 03:00 in their stored
 // timezone. The scheduler reconciles its in-memory job set with the
@@ -34,7 +33,7 @@ import (
 
 // dailyRebuildHour is the local-time hour at which each user's system
 // playlists rebuild. 03:00 is well past most "today" listening and
-// before morning use; matches the operator framing in the design doc.
+// before morning use.
 const dailyRebuildHour = 3
 
 // reconciliationInterval drives the hourly pass that brings the
@@ -48,7 +47,7 @@ type Scheduler struct {
 	pool    *pgxpool.Pool
 	logger  *slog.Logger
 	dataDir string
-	bus     *eventbus.Bus // #968: announce rebuilds so clients self-refresh
+	bus     *eventbus.Bus // announces rebuilds so clients self-refresh
 
 	mu   sync.Mutex
 	jobs map[pgtype.UUID]uuid.UUID // user_id → gocron job id
@@ -245,10 +244,10 @@ func (s *Scheduler) rebuildUserDaily(ctx context.Context, userID pgtype.UUID, no
 			"user_id", uuidStringPL(userID), "err", err)
 		return
 	}
-	// #968: tell the user's connected clients their home content was
-	// regenerated, so a tab/app left open across the rebuild refreshes its
-	// system-playlist + You-might-like views (and a stale active queue can
-	// re-pull) instead of serving yesterday's snapshot until a manual reload.
+	// Tell the user's connected clients their home content was regenerated, so
+	// a tab/app left open across the rebuild refreshes its system-playlist +
+	// You-might-like views (and a stale active queue can re-pull) instead of
+	// serving yesterday's snapshot until a manual reload.
 	s.publishRebuilt(userID)
 }