Bring the architecture doc back up to date with this session's changes

Closes both open "Documentation maintenance" roadmap items. Used a research-only
Explore pass to get a concrete diff against the actual code before editing, rather
than guessing what was stale.

Documented: src/track_layouts_logic.js (previously unmentioned anywhere),
buildEventSchedule, the Settings/Event Manager tab layouts, new state fields
(raceConfig.scheduleStartAt/scheduleGapMin, Session.trackLayoutId, top-level
trackLayouts/audioEventLog), a real SQLite table contract section, a full
field-by-field Settings shape, several previously-undocumented backend routes,
and the complete list of files tests/browser_smoke.js actually invokes.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "rc-racecore",
-  "version": "1.20.1",
+  "version": "1.20.2",
   "private": true,
   "description": "RC RaceCore live timing and race control with AMMC ingest and SQLite persistence",
   "engines": {