v0.12.3

What's Changed

• SSE hardening: queue-based writer with backpressure and directory-aware batching for improved reliability
• SSE frame utility: added structured SSE framing utility for consistent event formatting
• Parts batcher: defer operations when target message is missing from cache to avoid race conditions
• Model store: skip unnecessary subscriber notifications on no-op variant changes
• TypeScript: fix type narrowing error on messages.length check during loading state
• Docker: update Node.js base image and docker-compose config improvements
• Docker upgrade script: improved upgrade workflow with better error handling