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>

backend/tests/test_smoke.py

@@ -0,0 +1,31 @@
+"""Smoke tests that verify the test infrastructure is working correctly."""
+
+
+async def test_games_endpoint_returns_empty_list(client):
+    """Games endpoint returns an empty list on a clean database."""
+    response = await client.get("/api/v1/games")
+    assert response.status_code == 200
+    assert response.json() == []
+
+
+async def test_runs_endpoint_returns_empty_list(client):
+    """Runs endpoint returns an empty list on a clean database."""
+    response = await client.get("/api/v1/runs")
+    assert response.status_code == 200
+    assert response.json() == []
+
+
+async def test_pokemon_endpoint_returns_empty_list(client):
+    """Pokemon endpoint returns paginated empty result on a clean database."""
+    response = await client.get("/api/v1/pokemon")
+    assert response.status_code == 200
+    data = response.json()
+    assert data["items"] == []
+    assert data["total"] == 0
+
+
+async def test_database_isolation_between_tests(client):
+    """Confirm state from previous tests does not leak into this one."""
+    response = await client.get("/api/v1/games")
+    assert response.status_code == 200
+    assert response.json() == []