Security Hardening & Reliability Improvements

Comprehensive security audit and fixes across the platform. Stronger input validation, improved webhook protection, better error handling for long-running tasks, and startup health checks ensure your data and workflows are safer than ever.

• ›Stronger input validation -- message size limits and file name sanitization prevent abuse and protect system stability
• ›Improved webhook security -- replay attack prevention with timestamp verification ensures only legitimate events are processed
• ›Better billing protection -- payment failure handling no longer incorrectly reactivates canceled subscriptions
• ›Smarter file uploads -- workspace-to-chat ownership verification prevents cross-workspace data access
• ›Stream reliability -- 15-minute timeout on long-running streams prevents indefinite hangs
• ›Startup health checks -- critical configuration is validated at server startup, not at first request
• ›Safer background tasks -- atomic task creation prevents race conditions when sending rapid messages