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/recommendation/suggestions_select_test.go

@@ -8,7 +8,7 @@ import (
 )
 
 // selectSuggestions is deliberately pure (no DB, no clock), so the rotation and
-// diversity rules from slice #2373 are covered here in the fast lane rather
+// diversity rules are covered here in the fast lane rather
 // than behind the integration gate.
 
 func seedID(n byte) pgtype.UUID {
@@ -95,13 +95,12 @@ func TestSelectSuggestions_HeadIsStableTopScorers(t *testing.T) {
 	}
 }
 
-// #2367 mechanism 4: all twelve slots could be neighbours of one artist.
+// Twelve slots must not all be neighbours of one artist.
 //
 // The pool is deliberately SKEWED — one seed owns the entire top of the score
 // ranking — because that is the only shape where a cap can bite. With scores
 // spread evenly across seeds the top-N is already diverse and the cap is
-// untestable (an earlier version of this test asserted against an even pool and
-// could not fail).
+// untestable: an even pool cannot fail this test.
 func TestSelectSuggestions_CapsOneDominantSeed(t *testing.T) {
 	const dominantRun = 20
 	pool := make([]ArtistSuggestion, 0, 48)
@@ -131,7 +130,7 @@ func TestSelectSuggestions_CapsOneDominantSeed(t *testing.T) {
 	}
 }
 
-// Diversity must not starve the deck (rule #131): a pool hanging off a single
+// Diversity must not starve the deck: a pool hanging off a single
 // seed should still fill, not collapse to maxPerSeed entries.
 func TestSelectSuggestions_SingleSeedPoolStillFills(t *testing.T) {
 	got := selectSuggestions(poolOf(30, 1), 12, "2026-08-01")