fix: async I/O, map iteration bug, unified cleanup, Zod validation, error handling

- Convert sync readFileSync/statSync to async in subagent-watcher.ts to
  unblock event loop on hot paths (transcript reads, liveness checks)
- Fix Map mutation during iteration in closeSessionLogViewerWindows and
  closeSessionImagePopups (collect IDs first, then iterate to close)
- Unify session cleanup into shared _cleanupSessionData() method called
  from both closeSession() and session:deleted handler to prevent leaks
- Add Zod validation schemas for 20+ API routes that used raw type casts
- Add consecutive error tracking (5 errors/60s triggers exit for systemd
  restart) and SIGHUP handler for SSH disconnect safety

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

src/index.ts

@@ -14,12 +14,29 @@ import { program } from './cli.js';
 // In web mode, we should NOT exit on transient errors — log and continue
 const isWebMode = process.argv.includes('web');
 
+// Track consecutive unhandled errors in web mode — restart after too many
+let consecutiveErrors = 0;
+const MAX_CONSECUTIVE_ERRORS = 5;
+const ERROR_RESET_MS = 60000; // Reset counter after 1 minute of no errors
+let errorResetTimer: ReturnType<typeof setTimeout> | null = null;
+
+function trackError(): void {
+  consecutiveErrors++;
+  if (errorResetTimer) clearTimeout(errorResetTimer);
+  errorResetTimer = setTimeout(() => { consecutiveErrors = 0; }, ERROR_RESET_MS);
+
+  if (consecutiveErrors >= MAX_CONSECUTIVE_ERRORS) {
+    console.error(`[FATAL] ${MAX_CONSECUTIVE_ERRORS} consecutive unhandled errors — exiting for systemd restart`);
+    process.exit(1);
+  }
+}
+
 // Handle uncaught errors
 process.on('uncaughtException', (err) => {
   console.error('Uncaught exception:', err.message);
   if (isWebMode) {
-    // Log full stack trace for debugging but keep the server running
     console.error('[RECOVERED] Server continuing after uncaught exception:', err.stack);
+    trackError();
   } else {
     process.exit(1);
   }
@@ -29,6 +46,7 @@ process.on('unhandledRejection', (reason) => {
   console.error('Unhandled rejection:', reason);
   if (isWebMode) {
     console.error('[RECOVERED] Server continuing after unhandled rejection');
+    trackError();
   } else {
     process.exit(1);
   }