[QA] ✅ VERIFIED - All tests passing (262/262), manual API testing confirms correct behavior. Implementation properly validates reasoning_effort parameter using Literal types, correctly passes to Perplexity API only for sonar-deep-research model, and includes comprehensive test coverage. No security or quality issues found. Full verification report: /tmp/qa-PPLX-60.md

## Implementation Complete ### Changes Made 1. **backend/schemas.py:27** - Added Literal validation for reasoning_effort ('low', 'medium', 'high'). Invalid values now return HTTP 422. 2. **backend/perplexity_client.py:102-152** - Updated stream_query() to accept reasoning_effort and pass it to Perplexity API for sonar-deep-research model. 3. **backend/routers/queries.py:149** - Updated query router to pass reasoning_effort to stream_query(). 4. **backend/tests/test_async_queries.py:419-478** - Added tests for invalid value rejection (422) and API parameter passing. ### Verification - All 262 backend tests pass - Invalid value validation confirmed with 422 response ### Acceptance Criteria - [x] Can specify reasoning_effort in query request - [x] Parameter passed to Perplexity API correctly - [x] Invalid values rejected (422) - [x] Backend tests cover all effort levels