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/config.py

@@ -57,6 +57,9 @@ class Settings(BaseSettings):
     PRIVACY_SENSITIVE_OPERATIONS: str = ""  # comma-separated, e.g. "MailItemsAccessed,Search-Mailbox,Send"
     PRIVACY_SERVICE_ROLES: str = ""  # comma-separated, e.g. "SecurityAdministrator,ComplianceAdministrator"
 
+    # Redis (caching + async job queue)
+    REDIS_URL: str = "redis://localhost:6379/0"
+
 
 _settings = Settings()
 
@@ -95,3 +98,5 @@ LLM_API_VERSION = _settings.LLM_API_VERSION
 PRIVACY_SERVICES = {s.strip() for s in _settings.PRIVACY_SERVICES.split(",") if s.strip()}
 PRIVACY_SENSITIVE_OPERATIONS = {o.strip() for o in _settings.PRIVACY_SENSITIVE_OPERATIONS.split(",") if o.strip()}
 PRIVACY_SERVICE_ROLES = {r.strip() for r in _settings.PRIVACY_SERVICE_ROLES.split(",") if r.strip()}
+
+REDIS_URL = _settings.REDIS_URL