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/system_test.go

@@ -115,7 +115,7 @@ func TestBuildSystemPlaylists_SufficientActivity(t *testing.T) {
 				t.Errorf("songs_like_artist row should have non-NULL seed_artist_id")
 			}
 		case "discover", "deep_cuts", "rediscover", "new_for_you", "on_this_day", "first_listens":
-			// Discover + the #411 discovery mixes are all seedless —
+			// Discover and the discovery mixes are all seedless —
 			// no seed_artist_id required.
 		default:
 			t.Errorf("unknown system_variant=%q", *r.SystemVariant)
@@ -124,7 +124,7 @@ func TestBuildSystemPlaylists_SufficientActivity(t *testing.T) {
 			t.Errorf("row %s: empty track_count", uuidString(r.ID))
 		}
 		// For-You / Discover / the discovery mixes are sized to ~100
-		// (#352/#411); songs_like_artist stays at systemMixLength (25).
+		// songs_like_artist stays at systemMixLength (25).
 		if r.TrackCount > 100 {
 			t.Errorf("row %s: track_count=%d exceeds 100", uuidString(r.ID), r.TrackCount)
 		}
@@ -138,10 +138,10 @@ func TestBuildSystemPlaylists_SufficientActivity(t *testing.T) {
 }
 
 func TestBuildSystemPlaylists_StaleActivity_SongsLikeSurvives(t *testing.T) {
-	// #1255: the seed-artist query's old hard 7-day window emptied the
-	// pool after a quiet week, and the atomic-replace build then deleted
-	// every "Songs like X" mix. With the tiered fallback, plays that are
-	// ~20 days old (outside 7d, inside 30d) must still seed the mixes —
+	// A hard 7-day seed window would empty the pool after a quiet week, and
+	// the atomic-replace build would then delete every "Songs like X" mix.
+	// With the tiered fallback, plays that are ~20 days old (outside 7d,
+	// inside 30d) must still seed the mixes —
 	// and the built tracks carry the tier2 stamp so metrics can compare
 	// stale-seeded mixes against fresh ones.
 	pool := newPool(t)
@@ -352,16 +352,12 @@ func seedSharedArtistLibrary(
 }
 
 // The seed artist's own tracks are ELIGIBLE for its "Songs like" mix, and are
-// bounded by the diversity cap rather than excluded outright (#3881).
-//
-// produceSeedMixes used to filter them out — "Songs like X excludes X's own
-// songs" — which threw away the nearest neighbours of the seed TRACK and then
-// reached further out to replace them. Operator, 2026-09-10: "it should also
-// be able to include music from the same artist."
+// bounded by the diversity cap rather than excluded outright: excluding them
+// would throw away the nearest neighbours of the seed TRACK.
 //
 // Asserted end-to-end rather than by reading the source, because the guard has
-// to survive the filter coming back in a different shape — and because an
-// absence check would now match the comment explaining why the filter is gone.
+// to survive a filter coming back in a different shape — and because an
+// absence check would match the comment explaining why there is no filter.
 func TestBuildSystemPlaylists_SongsLikeIncludesItsSeedArtist(t *testing.T) {
 	pool := newPool(t)
 	logger := discardLogger()
@@ -403,7 +399,7 @@ func TestBuildSystemPlaylists_SongsLikeIncludesItsSeedArtist(t *testing.T) {
 		}
 		// The bound is what makes inclusion safe. Without it, "include the
 		// seed artist" becomes "the mix is mostly the seed artist", which is
-		// the radio failure (#3882) arriving on a different surface.
+		// the one-artist radio arriving on a different surface.
 		if own > maxPerArtist {
 			t.Errorf("a songs_like mix carries %d tracks by its own seed artist "+
 				"out of %d; the per-artist cap (%d) is not being applied",