[FEAT]: Async Background Task Queue for PDF Processing

[krrishrastogi05]

📝 Description

PDF fill operations in FireForm are currently synchronous and block the API thread for the entire duration of the request. This becomes a critical bottleneck under concurrent load — e.g., multiple departments submitting forms simultaneously.

💡 Rationale

For FireForm to scale as a Digital Public Good, it cannot afford to have a single long-running PDF fill block the server. An async job system decouples submission from processing, enabling parallel workloads and a better UX via status polling.

🛠️ Proposed Solution

• Introduce an async job queue using arq or asyncio background tasks integrated with FastAPI's BackgroundTasks
• On form submission, immediately return a job_id with a 202 Accepted response
• Add a GET /api/v1/jobs/{job_id} status endpoint returning { status, result, error }
• Store job state in Redis (for arq) or in-memory (for lightweight asyncio approach)
• Update Docker Compose to include Redis if needed

✅ Acceptance Criteria

• POST /api/v1/fill returns { job_id, status: "queued" } immediately
• GET /api/v1/jobs/{job_id} returns current status (queued, processing, complete, failed)
• Multiple simultaneous submissions are processed concurrently without blocking
• Works within the existing Docker container setup
• Testable via Swagger UI (/docs)