Set up backend test infrastructure

Add pytest fixtures (engine, db_session, client) with session-scoped
event loop to avoid asyncpg loop mismatch errors. Smoke tests verify
all three main API endpoints return empty results on a clean DB.
Test DB provided by docker-compose.test.yml on port 5433.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.beans/nuzlocke-tracker-yzpb--implement-unit-integration-tests.md

@@ -1,11 +1,11 @@
 ---
 # nuzlocke-tracker-yzpb
 title: Implement Unit & Integration Tests
-status: draft
+status: todo
 type: epic
 priority: high
 created_at: 2026-02-10T09:32:47Z
-updated_at: 2026-02-10T12:05:43Z
+updated_at: 2026-02-21T11:29:02Z
 ---
 
 Add comprehensive unit and integration test coverage to both the backend (FastAPI/Python) and frontend (React/TypeScript). The project currently has zero tests — pytest is configured in pyproject.toml with pytest-asyncio and httpx, but no actual test files exist. The frontend has no test tooling at all.
@@ -20,7 +20,7 @@ Add comprehensive unit and integration test coverage to both the backend (FastAP
 
 ## Success Criteria
 
-- [ ] Backend test infrastructure is set up (conftest, fixtures, test DB)
+- [x] Backend test infrastructure is set up (conftest, fixtures, test DB)
 - [ ] Backend schemas and services have unit test coverage
 - [ ] Backend API endpoints have integration test coverage
 - [ ] Frontend test infrastructure is set up (Vitest, RTL)