background_jobs: queue, workers, and enqueue path

[leseb]

Parent

Part of #32

Scope

• Filter skeleton: module, config, registry registration, on_request
• Bounded channel queue (capacity configurable, default 100)
• Long-lived tokio worker tasks (configurable count, default 10) pulling from queue
• On background=true: store response with status=queued, return immediately with queued response object
• Worker picks up request, runs full filter chain, updates store on completion
• Per-request timeout (configurable, default 5 minutes)
• Handle QueueFull → reject (no orphaned queued responses in storage)
• Polling: clients GET the response to check status
• Unit + integration tests

Why first

The queue and worker infrastructure is the foundation. Cancellation (#504) depends on being able to reference queued/in-progress worker tasks.

Reference

• Proposal: Responses API Filters — Filter 13
• Part of Epic: Responses API and Agentic Loop Orchestration #93

[shaneutt]

This needs to be coordinated with praxis-proxy/praxis#807 (we might even want to consider it a duplicate)