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/redis_client.py

@@ -0,0 +1,36 @@
+"""Async Redis client singleton for caching and job queue."""
+
+import redis.asyncio as aioredis
+from arq import create_pool
+from arq.connections import ArqRedis, RedisSettings
+from config import REDIS_URL
+
+_arq_pool: ArqRedis | None = None
+_plain_redis: aioredis.Redis | None = None
+
+
+async def get_arq_pool() -> ArqRedis:
+    """Return a shared arq pool (ArqRedis extends redis.asyncio.Redis)."""
+    global _arq_pool
+    if _arq_pool is None:
+        _arq_pool = await create_pool(RedisSettings.from_dsn(REDIS_URL))
+    return _arq_pool
+
+
+async def get_redis() -> aioredis.Redis:
+    """Return a shared plain async Redis client."""
+    global _plain_redis
+    if _plain_redis is None:
+        _plain_redis = aioredis.from_url(REDIS_URL, decode_responses=True)
+    return _plain_redis
+
+
+async def close_redis_connections():
+    """Close all Redis connections (call on shutdown)."""
+    global _arq_pool, _plain_redis
+    if _arq_pool:
+        await _arq_pool.close()
+        _arq_pool = None
+    if _plain_redis:
+        await _plain_redis.close()
+        _plain_redis = None