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/tagoverlap.go

@@ -1,13 +1,10 @@
-// tagoverlap.go — the taste-tag term for the Discover request surface
-// (#2377, milestone #268 slice 6).
+// tagoverlap.go — the taste-tag term for the Discover request surface.
 //
-// Slices 1-2 made the deck stop repeating; this is what makes it *relevant*.
-// Before this, a candidate's only claim on a slot was "some artist you play is
-// similar to it" — a graph-adjacency fact that says nothing about whether the
-// music sounds like anything you actually like. Here the candidate's own
-// folksonomy tags (cached by slice 5) are compared against the user's
-// taste-profile tags, so the surface can rank on "matches the sound you like"
-// and say WHY.
+// Similarity alone says "some artist you play is similar to it", a
+// graph-adjacency fact that says nothing about whether the music sounds like
+// anything you actually like. Here the candidate's own folksonomy tags (from
+// the candidate tag cache) are compared against the user's taste-profile
+// tags, so the surface can rank on "matches the sound you like" and say WHY.
 //
 // Pure by design — no DB, no clock — so the scoring rules are unit-testable in
 // the fast lane rather than behind the integration gate.
@@ -42,9 +39,8 @@ type TagWeights map[string]float64
 //
 // Returns (0, nil) when either side is empty. That is the load-bearing
 // degradation path: tag coverage for out-of-library candidates is permanently
-// partial (#2376), and a cold-start user has no taste tags at all. Both must
-// leave the candidate's similarity score untouched rather than sink it —
-// rule #131, tiered degradation, never vanish-or-nothing.
+// partial, and a cold-start user has no taste tags at all. Both must leave
+// the candidate's similarity score untouched rather than sink it.
 func tagOverlap(candidate, taste TagWeights) (float64, []string) {
 	if len(candidate) == 0 || len(taste) == 0 {
 		return 0, nil
@@ -123,7 +119,7 @@ func tagOverlap(candidate, taste TagWeights) (float64, []string) {
 //     weight would let a near-zero-similarity artist outrank a strong match
 //     just for sharing a popular tag, which reads as noise to the user.
 //   - weight 0 disables the feature completely and restores pure similarity
-//     order, so the operator's knob has a real off position.
+//     order, so the knob has a real off position.
 //
 // Callers must pass the pool in similarity order; it is returned in blended
 // order. Mutates the elements in place (they're the caller's own slice built
@@ -132,8 +128,8 @@ func tagOverlap(candidate, taste TagWeights) (float64, []string) {
 func applyTagOverlap(
 	pool []ArtistSuggestion, candidateTags map[string]TagWeights, taste TagWeights, weight float64,
 ) []ArtistSuggestion {
-	// A zero weight is the operator turning the feature off. Skip the work
-	// AND the re-sort so the ordering is bit-for-bit the pre-slice-6 result.
+	// A zero weight turns the feature off. Skip the work AND the re-sort so
+	// the ordering is bit-for-bit pure similarity order.
 	if weight == 0 || len(taste) == 0 {
 		return pool
 	}
@@ -149,7 +145,7 @@ func applyTagOverlap(
 		}
 		// Stable tiebreak by MBID. Without it, two candidates on equal scores
 		// could swap between requests within the same day, which the daily
-		// rotation (#2373) exists to prevent.
+		// rotation exists to prevent.
 		return pool[i].MBID < pool[j].MBID
 	})
 	return pool