fix: MEDIUM issues #23-#30 — stale deps, export modal, cache race, StrictMode dedup, SeatingService ref, SVG import persist, group-as-block multi-select, session expiry

backend/src/database/SqliteAdapter.ts

@@ -267,7 +267,10 @@ export class SqliteAdapter implements DatabaseInterface {
   }
 
   deleteExpiredSessions(): void {
-    this.db.prepare('DELETE FROM sessions WHERE expires_at < ?').run(Date.now());
+    // Use consistent epoch-millisecond integer comparison.
+    // <= ensures sessions expiring at exactly the current time are also deleted.
+    const now = Math.floor(Date.now());
+    this.db.prepare('DELETE FROM sessions WHERE expires_at <= ?').run(now);
   }
 
   // ─── Notifications ──────────────────────────────────