WebSocket cost_so_far updates during generation

[rancur]

Context

The cost guard tracks running cost during generation, but WebSocket updates don't yet include cost_so_far.

Work needed

• In server/services/job_queue.py, pass cost_so_far in ws_manager.send_job_update() calls
• The pipeline needs a callback mechanism to report cost after each segment
• Frontend GenerationQueue page should display running cost next to progress

Related

Part of the cost protection system added in 235ce12.