docs: api comments say what the endpoints do, not how they came to be (#6277)

Rewrites the internal/api comment blocks to rule 32: task, milestone and
rule ids gone, the stories behind them restated as the constraint they
left, stale references corrected (the stream secret is wired from config,
the timezone handler already refreshes the scheduler, the Android app is
the native client, the design docs cited are not in the repo). Also
rewraps comment lines over 80 columns left by the earlier batches.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/api/metrics_significance_test.go

@@ -6,9 +6,8 @@ import (
 )
 
 func TestProportionDelta_ReproducesTheDiscoverCase(t *testing.T) {
-	// The comparison that motivated #2495: Discover taste-matched (59 plays,
-	// 15.3% skip) vs random-unheard (70 plays, 28.6%). A 13.3pp gap that the old
-	// card rendered as a confident coloured number, sitting at p ≈ 0.06.
+	// 59 plays at 15.3% skip vs 70 at 28.6%: a 13.3pp gap that looks
+	// confident and sits at p ≈ 0.06.
 	d := proportionDelta(0.153, 59, 0.286, 70)
 	if d == nil {
 		t.Fatal("expected a delta for two real samples")
@@ -16,8 +15,8 @@ func TestProportionDelta_ReproducesTheDiscoverCase(t *testing.T) {
 	if math.Abs(d.DeltaPP-(-13.3)) > 0.1 {
 		t.Errorf("DeltaPP = %.2f, want ≈ -13.3", d.DeltaPP)
 	}
-	// This is the assertion the whole task exists for: at these sample sizes the
-	// margin swallows the difference.
+	// This is the assertion the whole task exists for: at these sample sizes
+	// the margin swallows the difference.
 	if d.Distinguishable {
 		t.Errorf("13.3pp on n=59/70 reported as distinguishable (margin %.2f) — "+
 			"this is exactly the false confidence #2495 set out to remove", d.MarginPP)
@@ -70,7 +69,8 @@ func TestProportionDelta_DegenerateRates(t *testing.T) {
 	if d := proportionDelta(1, 50, 1, 50); d != nil {
 		t.Errorf("both rates 1 produced a delta: %+v", d)
 	}
-	// One degenerate side is still informative — the other side carries variance.
+	// One degenerate side is still informative — the other side carries
+	// variance.
 	if d := proportionDelta(0, 200, 0.3, 200); d == nil {
 		t.Error("one degenerate rate should still yield a delta")
 	}