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/tmux-manager.ts

@@ -141,6 +141,7 @@ export class TmuxManager extends EventEmitter implements TerminalMultiplexer {
 
   constructor() {
     super();
+    this.setMaxListeners(50);
     if (!IS_TEST_MODE) {
       this.loadSessions();
     }