Show real-time processing progress via WebSockets

[jeisaacs]

Summary

Expose real-time processing updates from the backend so users see live progress instead of a static spinner.

Why

• Long-running jobs feel unresponsive today
• Detailed status helps debug stalled or failed runs
• Unlocks richer UX elements like per-page progress bars

Proposal

• Establish a WebSocket channel (or Server-Sent Events) that streams processing milestones
• Emit granular events: upload complete, parsing, interleaving, writing output, success/failure
• Update the frontend modal/progress indicator as events arrive

Acceptance Criteria

• Backend broadcasts processing lifecycle events per job
• Frontend subscribes and renders incremental progress, including errors
• Activity log and UI stay in sync with streamed events
• Graceful fallback when real-time channel is unavailable