fix: medium severity audit fixes — races, leaks, perf, warnings

- Clear teams/teamTasks maps in handleInit() on SSE reconnect
- Fix saveNowAsync() race condition with in-flight promise guard
- Add setMaxListeners() on Session, WebServer, SubagentWatcher,
  ImageWatcher, TmuxManager to prevent MaxListenersExceeded warnings
- Debounce persistSessionState() per-session (100ms) to reduce
  redundant toState() serialization across 25+ call sites
- Cache getLightSessionsState() with 1s TTL to avoid re-serializing
  all sessions on every SSE connect and /api/sessions request

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/web/public/app.js

@@ -3097,6 +3097,8 @@ class ClaudemanApp {
     this.terminalBuffers.clear();
     this.terminalBufferCache.clear();
     this.projectInsights.clear();
+    this.teams.clear();
+    this.teamTasks.clear();
     // Clear all idle timers to prevent stale timers from firing
     for (const timer of this.idleTimers.values()) {
       clearTimeout(timer);