diff --git a/backend/tests/conftest.py b/backend/tests/conftest.py
new file mode 100644
index 0000000..9abe1b8
--- /dev/null
+++ b/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()
diff --git a/backend/tests/test_core_api.py b/backend/tests/test_core_api.py
new file mode 100644
index 0000000..5da62cb
--- /dev/null
+++ b/backend/tests/test_core_api.py
@@ -0,0 +1,91 @@
+"""End-to-end coverage of the core data model through the API: tenancy, the
+privacy seam, and the pre-auth actor shim."""
+
+
+async def _create_user(client, email: str) -> str:
+    resp = await client.post(
+        "/api/v1/users", json={"email": email, "display_name": email}
+    )
+    assert resp.status_code == 201, resp.text
+    return resp.json()["id"]
+
+
+async def test_tree_and_person_flow(client):
+    user_id = await _create_user(client, "keeper@example.com")
+    headers = {"X-User-Id": user_id}
+
+    resp = await client.post(
+        "/api/v1/trees", json={"name": "Smith Family", "visibility": "private"}, headers=headers
+    )
+    assert resp.status_code == 201, resp.text
+    tree = resp.json()
+    assert tree["visibility"] == "private"
+    assert tree["owner_id"] == user_id
+    tree_id = tree["id"]
+
+    resp = await client.get("/api/v1/trees", headers=headers)
+    assert resp.status_code == 200
+    assert len(resp.json()) == 1
+
+    resp = await client.post(
+        f"/api/v1/trees/{tree_id}/persons",
+        json={"given": "John", "surname": "Smith"},
+        headers=headers,
+    )
+    assert resp.status_code == 201, resp.text
+    person = resp.json()
+    assert person["primary_name"] == "John Smith"
+    assert person["tree_id"] == tree_id
+
+    resp = await client.get(f"/api/v1/trees/{tree_id}/persons", headers=headers)
+    assert resp.status_code == 200
+    assert len(resp.json()) == 1
+
+
+async def test_private_tree_isolated_from_other_users(client):
+    owner = await _create_user(client, "owner@example.com")
+    other = await _create_user(client, "stranger@example.com")
+
+    resp = await client.post(
+        "/api/v1/trees", json={"name": "Private", "visibility": "private"},
+        headers={"X-User-Id": owner},
+    )
+    tree_id = resp.json()["id"]
+
+    # A non-member cannot view a private tree, nor list its people.
+    resp = await client.get(f"/api/v1/trees/{tree_id}", headers={"X-User-Id": other})
+    assert resp.status_code == 403
+    resp = await client.get(f"/api/v1/trees/{tree_id}/persons", headers={"X-User-Id": other})
+    assert resp.status_code == 403
+
+
+async def test_public_tree_viewable_but_not_editable_by_non_member(client):
+    owner = await _create_user(client, "owner2@example.com")
+    viewer = await _create_user(client, "viewer2@example.com")
+
+    resp = await client.post(
+        "/api/v1/trees", json={"name": "Public", "visibility": "public"},
+        headers={"X-User-Id": owner},
+    )
+    tree_id = resp.json()["id"]
+
+    # Visible to a non-member...
+    resp = await client.get(f"/api/v1/trees/{tree_id}", headers={"X-User-Id": viewer})
+    assert resp.status_code == 200
+    # ...but not writable (not an editor).
+    resp = await client.post(
+        f"/api/v1/trees/{tree_id}/persons", json={"given": "Nope"},
+        headers={"X-User-Id": viewer},
+    )
+    assert resp.status_code == 403
+
+
+async def test_duplicate_email_conflicts(client):
+    await _create_user(client, "dupe@example.com")
+    resp = await client.post("/api/v1/users", json={"email": "dupe@example.com"})
+    assert resp.status_code == 409
+
+
+async def test_auth_required_without_header(client):
+    resp = await client.get("/api/v1/trees")
+    assert resp.status_code == 401