feat: Redis caching + async queue for LLM scaling (v1.6.0)

- Add async Redis client singleton (redis_client.py) for caching and arq pool
- Add arq job functions (jobs.py) for background LLM processing
- Cache ask/explain LLM responses with TTL (1h ask, 24h explain)
- Add async mode to /api/ask: enqueue job, return job_id, poll /api/jobs/{id}
- Add GET /api/jobs/{job_id} endpoint for job status polling
- Add arq worker service to docker-compose (dev + prod)
- Switch from Redis to Valkey (BSD fork) in Docker Compose
- Add REDIS_URL config setting
- Add tests for cache hit, async mode, and job status

backend/routes/jobs.py

@@ -0,0 +1,43 @@
+"""Job status endpoints for async LLM operations."""
+
+from arq.jobs import Job, JobStatus
+from auth import require_auth
+from fastapi import APIRouter, Depends, HTTPException
+from pydantic import BaseModel
+from redis_client import get_redis
+
+router = APIRouter(dependencies=[Depends(require_auth)])
+
+
+class JobStatusResponse(BaseModel):
+    job_id: str
+    status: str  # queued, in_progress, complete, not_found, deferred
+    result: dict | None = None
+    error: str | None = None
+
+
+@router.get("/jobs/{job_id}", response_model=JobStatusResponse)
+async def get_job_status(job_id: str, user: dict = Depends(require_auth)):
+    """Poll for the result of an async LLM job."""
+    redis = await get_redis()
+    job = Job(job_id, redis)
+    status = await job.status()
+
+    if status == JobStatus.not_found:
+        raise HTTPException(status_code=404, detail="Job not found")
+
+    result = None
+    error = None
+    if status == JobStatus.complete:
+        try:
+            result_data = await job.result(timeout=0)
+            result = result_data if isinstance(result_data, dict) else {"data": str(result_data)}
+        except Exception as exc:
+            error = str(exc)
+
+    return JobStatusResponse(
+        job_id=job_id,
+        status=status.value,
+        result=result,
+        error=error,
+    )