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/discover_taste_db_test.go

@@ -29,11 +29,11 @@ func seedTasteTag(t *testing.T, pool *pgxpool.Pool, userID pgtype.UUID, tag stri
 	}
 }
 
-// TestListTasteUnheardTracksForDiscover_CapsBeforeLimit is the deploy's shape
-// (#5356): one artist whose every track carries the whole taste profile
-// outscores everything on summed weight. Capped only after the LIMIT, that
-// artist took all 120 rows and the bucket shrank to its 3. Capped before, the
-// artist keeps 3 and the single-tag artists below it fill the rest.
+// TestListTasteUnheardTracksForDiscover_CapsBeforeLimit: one artist whose every
+// track carries the whole taste profile outscores everything on summed
+// weight. Capped only after the LIMIT, that artist would take all 120 rows
+// and the bucket shrink to its 3. Capped before, the artist keeps 3 and the
+// single-tag artists below it fill the rest.
 func TestListTasteUnheardTracksForDiscover_CapsBeforeLimit(t *testing.T) {
 	pool := newPool(t)
 	ctx := context.Background()