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_integration_test.go

@@ -331,7 +331,7 @@ func TestSuggestArtists_EmptyForNewUser(t *testing.T) {
 	}
 }
 
-// --- Slice 1 (#2372): taste-profile seeding, tiering, and the skip fix ---
+// --- Taste-profile seeding, tiering, and skipped plays ---
 
 func setTasteWeight(t *testing.T, pool *pgxpool.Pool, userID, artistID pgtype.UUID, weight float64) {
 	t.Helper()
@@ -365,7 +365,7 @@ func insertSkippedPlayEvent(t *testing.T, pool *pgxpool.Pool, userID, trackID pg
 }
 
 // Tier 1: a taste-profile weight alone seeds a suggestion, with no like and no
-// play on the seed artist. Before #2367 the profile was ignored entirely here.
+// play on the seed artist.
 func TestSuggestArtists_TasteProfileWeightSeedsTier1(t *testing.T) {
 	pool := newPool(t)
 	user := seedUser(t, pool, "alice")
@@ -449,9 +449,9 @@ func TestSuggestArtists_NonPositiveTasteWeightDoesNotSeed(t *testing.T) {
 	}
 }
 
-// Tier 2 counts COMPLETED plays only. Previously every play_event counted, so
-// skipping an artist repeatedly increased its signal and pushed more of its
-// neighbours at the user (#2367 mechanism 3).
+// Tier 2 counts COMPLETED plays only. Counting every play_event would make
+// skipping an artist repeatedly increase its signal and push more of its
+// neighbours at the user.
 func TestSuggestArtists_SkippedPlaysDoNotSeedTier2(t *testing.T) {
 	pool := newPool(t)
 	user := seedUser(t, pool, "alice")
@@ -473,12 +473,11 @@ func TestSuggestArtists_SkippedPlaysDoNotSeedTier2(t *testing.T) {
 	}
 }
 
-// --- Slice 3 (#2374): time-boxed suggestion snooze ---
+// --- Time-boxed suggestion snooze ---
 //
-// The defining property under test is that a snooze EXPIRES. A permanent
-// dismissal would pass most of these; only TestSuggestArtists_ExpiredSnooze
-// distinguishes the two, and it is the reason this shape was approved over an
-// exclusion UI (rule #101).
+// The defining property under test is that a snooze EXPIRES: suggestions have
+// no permanent "never show" control. A permanent dismissal would pass most of
+// these; only TestSuggestArtists_ExpiredSnooze distinguishes the two.
 
 // snoozeUntil inserts a snooze row with an explicit absolute expiry, so a
 // test can place it in the past without depending on the duration arithmetic
@@ -546,7 +545,7 @@ func TestSuggestArtists_ExpiredSnoozeShowsCandidateAgain(t *testing.T) {
 	}
 }
 
-// Rule #47: one household member parking a suggestion must not remove it
+// One household member parking a suggestion must not remove it
 // from anyone else's deck.
 func TestSuggestArtists_SnoozeIsPerUser(t *testing.T) {
 	pool := newPool(t)