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/library_sync_views_test.go

@@ -81,9 +81,9 @@ func TestTrackSyncView_WireKeys(t *testing.T) {
 	})
 }
 
-// The mark is what the client filters on, so a wrong value here silently
-// re-introduces #2704: a present track marked missing vanishes from the
-// client's library, a missing one stays playable and fails at the speaker.
+// The mark is what the client filters on, so a wrong value here is silent: a
+// present track marked missing vanishes from the client's library, a missing
+// one stays playable and fails at the speaker.
 func TestTrackSyncView_MissingReflectsTheMark(t *testing.T) {
 	present := dbq.Track{ID: validUUID, AlbumID: validUUID, ArtistID: validUUID}
 	if toTrackSyncView(present, library.ReplayGain{}).Missing {
@@ -97,7 +97,7 @@ func TestTrackSyncView_MissingReflectsTheMark(t *testing.T) {
 	}
 }
 
-// The gains are what Android levels playback by, offline included (#4997):
+// The gains are what Android levels playback by, offline included:
 // the track's pair rides on the track view and the album's on the album view.
 func TestSyncViews_CarryReplayGain(t *testing.T) {
 	gain, peak := float32(-3.25), float32(0.9441)