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/you_might_like.go

@@ -1,5 +1,5 @@
-// you_might_like.go builds the per-user "You might like" Home rows
-// (#790): in-library albums/artists the listener doesn't actively spin
+// you_might_like.go builds the per-user "You might like" Home rows:
+// in-library albums/artists the listener doesn't actively spin
 // but is predicted to enjoy. It reuses the For-You candidate engine
 // (similarity + like-weighted scoring) and rolls the per-track scores up
 // to album/artist granularity, gated on a minimum listening history so a
@@ -84,7 +84,7 @@ func buildYouMightLike(
 		return youMightLikeResult{built: false}
 	}
 	// One rotating seed is right here (unlike For-You's multi-seed
-	// blend, #1269): the row is a short shelf, not a mix, and a single
+	// blend): the row is a short shelf, not a mix, and a single
 	// neighborhood per day keeps it coherent.
 	daily := pickDailySeeds(seeds, userID, dateStr, 1)
 	if len(daily) == 0 {