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/playlists_test.go

@@ -571,13 +571,15 @@ func TestListPlaylists_BadKindReturns400(t *testing.T) {
 }
 
 // TestEditEndpoints_403OnSystemPlaylist verifies that all five edit verbs
-// return 403 system_playlist_readonly when the target playlist has kind='system'.
+// return 403 system_playlist_readonly when the target playlist has
+// kind='system'.
 func TestEditEndpoints_403OnSystemPlaylist(t *testing.T) {
 	h, pool := testHandlers(t)
 	u := seedUser(t, pool, "sysedit", "pw", false)
 
 	// Seed a system playlist directly via SQL (BuildSystemPlaylists requires
-	// play_events seeding which we already exercise in playlists package tests).
+	// play_events seeding which we already exercise in playlists package
+	// tests).
 	var sid string
 	if err := pool.QueryRow(context.Background(), `
 		INSERT INTO playlists (user_id, name, description, is_public, kind, system_variant)