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/candidates_v2_test.go

@@ -70,7 +70,7 @@ func TestLoadCandidatesFromSimilarity_LBSimilarSourceContributes(t *testing.T) {
 	if found == nil {
 		t.Fatal("LB-similar target missing from candidates")
 	}
-	// Tier 1's band (#6177): 0.5 + 0.5 × 0.85.
+	// Tier 1's band: 0.5 + 0.5 × 0.85.
 	if found.Inputs.SimilarityScore < 0.92 || found.Inputs.SimilarityScore > 0.93 {
 		t.Errorf("LB-similar SimilarityScore = %v, want ~0.925 (0.5 + 0.5 × 0.85)", found.Inputs.SimilarityScore)
 	}
@@ -258,7 +258,7 @@ func TestLoadCandidatesFromSimilarity_DedupTakesMaxScore(t *testing.T) {
 		if c.Track.ID == target.ID {
 			count++
 			// max() keeps LB's 0.55. Even a weak ListenBrainz edge is tier 1 and
-			// outranks a full tag match (#6177).
+			// outranks a full tag match.
 			if c.Inputs.SimilarityScore < 0.54 || c.Inputs.SimilarityScore > 0.56 {
 				t.Errorf("dedup max SimilarityScore = %v, want ~0.55 (the LB edge's band should win)", c.Inputs.SimilarityScore)
 			}
@@ -269,7 +269,7 @@ func TestLoadCandidatesFromSimilarity_DedupTakesMaxScore(t *testing.T) {
 	}
 }
 
-// TestLoadCandidatesFromSimilarity_TasteOverlapArm (#796 phase 2b): a track by
+// TestLoadCandidatesFromSimilarity_TasteOverlapArm: a track by
 // a positively-weighted taste-profile artist enters the pool via taste_overlap
 // even with every other arm disabled; a negatively-weighted artist's track does
 // not (the WHERE weight > 0 filter).
@@ -339,22 +339,16 @@ func TestLoadCandidatesFromSimilarity_EmptyLibrary_NoError(t *testing.T) {
 	}
 }
 
-// The randomised arms must draw REPRODUCIBLY for a given seed (#3889).
+// The randomised arms must draw REPRODUCIBLY for a given seed.
 //
-// Four arms used to end in a bare `ORDER BY random()`. That returned a stable
-// set only while the arm's LIMIT exceeded the rows eligible for it — at that
-// point it returned all of them and the order stopped mattering, because the
-// caller sorts by track id before scoring. Below that threshold it returned a
-// random SUBSET, so two calls drew different candidates.
+// An arm ordered by bare `random()` returns a stable set only while its LIMIT
+// exceeds the rows eligible for it (it returns all of them, and the caller
+// sorts by track id). Below that it returns a random SUBSET, so two calls
+// draw different candidates — invisible, because a mix that changes after a
+// refresh looks like a feature.
 //
-// It therefore held by ACCIDENT, and only for libraries smaller than the
-// limits. Any real library is larger, so same-day rebuilds had been drawing
-// different mixes since the arm was written — invisible, because a mix that
-// changes after a refresh looks like a feature.
-//
-// Limits deliberately smaller than the fixture, because that is the only
-// regime where the bug existed at all: with limits above the eligible count
-// the old code passes this too.
+// The limits are deliberately smaller than the fixture: with limits above the
+// eligible count, an unseeded arm passes this too.
 func TestLoadCandidatesFromSimilarity_SameSeedDrawsTheSameSet(t *testing.T) {
 	f := newFixture(t, 12)
 	seed := f.tracks[0]
@@ -433,10 +427,9 @@ func TestLoadCandidatesFromSimilarity_DifferentSeedsCanDrawDifferently(t *testin
 	}
 }
 
-// #5297: an artist-level arm takes a track from each related artist before a
-// second from any. Ordered by artist score alone, the closest artist's
-// catalogue filled the whole LIMIT; on the operator's library the
-// similar_artists arm held one artist for every seed measured (#3879).
+// An artist-level arm takes a track from each related artist before a second
+// from any. Ordered by artist score alone, the closest artist's catalogue
+// would fill the whole LIMIT.
 func TestLoadCandidatesFromSimilarity_ArtistArmsRoundRobin(t *testing.T) {
 	for _, source := range []string{"listenbrainz", "user_cooccurrence"} {
 		t.Run(source, func(t *testing.T) {
@@ -500,8 +493,8 @@ func TestLoadCandidatesFromSimilarity_ArtistArmsRoundRobin(t *testing.T) {
 }
 
 // The taste arm takes one track from each taste artist before a second from
-// any (#3880). Ordered by weight then track id, the strongest artist's
-// catalogue filled the arm: 20 tracks of one artist on the operator's library.
+// any. Ordered by weight then track id, the strongest artist's catalogue would
+// fill the arm.
 func TestLoadCandidatesFromSimilarity_TasteArmRoundRobinsAcrossArtists(t *testing.T) {
 	f := newFixture(t, 1)
 	ctx := context.Background()
@@ -552,9 +545,9 @@ func TestLoadCandidatesFromSimilarity_TasteArmRoundRobinsAcrossArtists(t *testin
 	}
 }
 
-// Tracks tied on tag overlap are taken in the order seed's order (#3880): the
-// same seed takes the same slice, another seed another. With no tie-break a
-// one-tag seed's LIMIT took whichever rows Postgres reached first.
+// Tracks tied on tag overlap are taken in the order seed's order: the same
+// seed takes the same slice, another seed another. With no tie-break a
+// one-tag seed's LIMIT would take whichever rows Postgres reached first.
 func TestLoadCandidatesFromSimilarity_TagTiesFollowTheOrderSeed(t *testing.T) {
 	f := newFixture(t, 21)
 	ctx := context.Background()