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_mixes_test.go

@@ -21,7 +21,7 @@ func idsOf(pool []discoverTrack) []byte {
 func TestRotateForDay_RotatesWithinTierBlocks(t *testing.T) {
 	// Tiered pools arrive tier-ordered; rotation must vary the daily
 	// slice WITHIN each tier without hoisting tier-3 filler above
-	// tier-1's exact fits (#1267).
+	// tier-1's exact fits.
 	pool := []discoverTrack{
 		mixTrack(1, pickKindTier1), mixTrack(2, pickKindTier1), mixTrack(3, pickKindTier1),
 		mixTrack(4, pickKindTier2), mixTrack(5, pickKindTier2),
@@ -80,7 +80,7 @@ func TestRotateForDay_UntieredPoolIsWholeRotation(t *testing.T) {
 
 func TestFinishMix_PropagatesPickKind(t *testing.T) {
 	// Tier stamps ride discoverTrack through diversify/truncate into
-	// the rankedCandidates that insertSystemPlaylist persists (#1267).
+	// the rankedCandidates that insertSystemPlaylist persists.
 	pool := []discoverTrack{mixTrack(1, pickKindTier1), mixTrack(2, pickKindTier2)}
 	got := finishMix(pool, false)
 	if len(got) != 2 {