Add core API integration tests

End-to-end coverage of the tenancy/people flow and the privacy seam (private-tree isolation, public-tree view-but-not-edit, duplicate-email conflict, auth-required). DB-backed tests run against TEST_DATABASE_URL and skip cleanly when it is unset, so the no-DB suite still runs anywhere.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: Justin Paul <justin@jpaul.me>

backend/tests/conftest.py

@@ -0,0 +1,47 @@
+"""Test fixtures.
+
+DB-backed tests require ``TEST_DATABASE_URL`` (an async URL to a *disposable*
+Postgres); without it they skip, so the no-DB unit suite still runs anywhere.
+The schema is built from the models via ``create_all`` and dropped per test for
+isolation.
+"""
+
+import os
+
+import pytest_asyncio
+from httpx import ASGITransport, AsyncClient
+from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker, create_async_engine
+
+import app.models  # noqa: F401 — register all models on Base.metadata
+from app.core.db import get_session
+from app.main import app
+from app.models import Base
+
+TEST_DATABASE_URL = os.getenv("TEST_DATABASE_URL")
+
+
+@pytest_asyncio.fixture
+async def client():
+    if not TEST_DATABASE_URL:
+        import pytest
+
+        pytest.skip("TEST_DATABASE_URL not set")
+
+    engine = create_async_engine(TEST_DATABASE_URL)
+    async with engine.begin() as conn:
+        await conn.run_sync(Base.metadata.drop_all)
+        await conn.run_sync(Base.metadata.create_all)
+
+    sessionmaker = async_sessionmaker(engine, expire_on_commit=False, class_=AsyncSession)
+
+    async def _override_session():
+        async with sessionmaker() as session:
+            yield session
+
+    app.dependency_overrides[get_session] = _override_session
+    transport = ASGITransport(app=app)
+    async with AsyncClient(transport=transport, base_url="http://test") as http_client:
+        yield http_client
+
+    app.dependency_overrides.clear()
+    await engine.dispose()