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/library_scale_test.go

@@ -18,9 +18,7 @@ func TestScaleForLibrary_SmallLibrariesAreUntouched(t *testing.T) {
 	}
 }
 
-// The complaint, restated as a property: a bigger library must reach further
-// into itself. Operator, 2026-09-10 — "is the pool that we draw from somehow
-// scaled to the amount of music in the library".
+// A bigger library must reach further into itself.
 func TestScaleForLibrary_BigLibrariesGetABiggerPool(t *testing.T) {
 	base := DefaultCandidateSourceLimits()
 	small := ScaleForLibrary(base, libraryScaleReference)
@@ -86,9 +84,9 @@ func TestScaleForLibrary_GrowthIsBounded(t *testing.T) {
 	}
 }
 
-// Never below the base. The base limits are a floor, not a midpoint — and
-// #3889 makes this load-bearing rather than tidy: shrinking an arm ordered by
-// unseeded random() changes pool membership between same-day rebuilds.
+// Never below the base. The base limits are a floor, not a midpoint, and
+// shrinking an arm ordered by unseeded random() would change pool membership
+// between same-day rebuilds.
 func TestScaleForLibrary_NeverShrinksAnArm(t *testing.T) {
 	base := DefaultCandidateSourceLimits()
 	for _, size := range []int64{0, 1, 100, 4999, 5001, 1_000_000} {