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/conftest.py

@@ -0,0 +1,61 @@
+import os
+
+import pytest
+from httpx import ASGITransport, AsyncClient
+from sqlalchemy.ext.asyncio import async_sessionmaker, create_async_engine
+
+import app.models  # noqa: F401 — ensures all models register with Base.metadata
+from app.core.database import Base, get_session
+from app.main import app
+
+TEST_DATABASE_URL = os.getenv(
+    "TEST_DATABASE_URL",
+    "postgresql+asyncpg://postgres:postgres@localhost:5433/nuzlocke_test",
+)
+
+
+@pytest.fixture(scope="session")
+async def engine():
+    """Create the test engine and schema once for the entire session."""
+    eng = create_async_engine(TEST_DATABASE_URL, echo=False)
+    async with eng.begin() as conn:
+        await conn.run_sync(Base.metadata.create_all)
+    yield eng
+    async with eng.begin() as conn:
+        await conn.run_sync(Base.metadata.drop_all)
+    await eng.dispose()
+
+
+@pytest.fixture
+async def db_session(engine):
+    """
+    Provide a database session for a single test.
+
+    Overrides the FastAPI get_session dependency so endpoint handlers use the
+    same session. Truncates all tables after the test to isolate state.
+    """
+    session_factory = async_sessionmaker(engine, expire_on_commit=False)
+    session = session_factory()
+
+    async def override_get_session():
+        yield session
+
+    app.dependency_overrides[get_session] = override_get_session
+
+    yield session
+
+    await session.close()
+    app.dependency_overrides.clear()
+
+    async with engine.begin() as conn:
+        for table in reversed(Base.metadata.sorted_tables):
+            await conn.execute(table.delete())
+
+
+@pytest.fixture
+async def client(db_session):
+    """Async HTTP client wired to the FastAPI app with the test database session."""
+    async with AsyncClient(
+        transport=ASGITransport(app=app), base_url="http://test"
+    ) as ac:
+        yield ac