diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index cdcc835..09cc77c 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -33,6 +33,22 @@ jobs:
     defaults:
       run:
         working-directory: backend
+    services:
+      postgres:
+        image: postgres:16-alpine
+        env:
+          POSTGRES_USER: postgres
+          POSTGRES_PASSWORD: postgres
+          POSTGRES_DB: trainable_test
+        ports:
+          - 5432:5432
+        options: >-
+          --health-cmd "pg_isready -U postgres"
+          --health-interval 5s
+          --health-timeout 5s
+          --health-retries 10
+    env:
+      TEST_DATABASE_URL: postgresql+asyncpg://postgres:postgres@localhost:5432/trainable_test
     steps:
       - uses: actions/checkout@v4
       - uses: actions/setup-python@v5
diff --git a/backend/pytest.ini b/backend/pytest.ini
new file mode 100644
index 0000000..4abf6a6
--- /dev/null
+++ b/backend/pytest.ini
@@ -0,0 +1,18 @@
+[pytest]
+# Explicit + pinned pytest-asyncio config (see conftest.py for fixture setup).
+#
+# asyncio_default_fixture_loop_scope / asyncio_default_test_loop_scope =
+# "session" gives the whole test session ONE event loop instead of a fresh
+# one per test (the pytest-asyncio 1.x default is "function"). This matters
+# because db.py's `engine` (the SQLAlchemy async engine / connection pool)
+# is created once at import time. Against SQLite/aiosqlite that mismatch is
+# silently tolerated, but asyncpg's connections are strictly bound to the
+# event loop that created them — reusing the pooled engine from a second,
+# different per-test event loop raises
+# "got Future <Future ...> attached to a different loop". Pinning both
+# scopes to "session" keeps the engine and every test on the same loop, so
+# the suite runs unchanged against Postgres in CI (see
+# .github/workflows/ci.yml) as well as the SQLite default.
+asyncio_mode = strict
+asyncio_default_fixture_loop_scope = session
+asyncio_default_test_loop_scope = session
diff --git a/backend/tests/conftest.py b/backend/tests/conftest.py
index f3e953e..b417c64 100644
--- a/backend/tests/conftest.py
+++ b/backend/tests/conftest.py
@@ -22,8 +22,21 @@ def pytest_configure(config):
     )
 
 
-# Use in-memory SQLite for tests (no Postgres needed)
-os.environ["DATABASE_URL"] = "sqlite+aiosqlite://"
+# Use in-memory SQLite for tests by default (no Postgres needed). CI's
+# backend-test job runs a Postgres service container and sets
+# TEST_DATABASE_URL so the suite exercises real Postgres semantics —
+# Column(JSON) storage, FK ON DELETE CASCADE, and the hand-rolled
+# db._run_migrations — instead of only ever validating against an engine
+# we don't ship. See .github/workflows/ci.yml.
+#
+# Only the explicit TEST_DATABASE_URL opt-in is honored: an ambient
+# DATABASE_URL (e.g. pointing at a dev database in a developer's shell) is
+# deliberately overwritten, because the setup_db fixture drops all tables
+# after every test.
+if os.environ.get("TEST_DATABASE_URL"):
+    os.environ["DATABASE_URL"] = os.environ["TEST_DATABASE_URL"]
+else:
+    os.environ["DATABASE_URL"] = "sqlite+aiosqlite://"
 
 # Mock claude_agent_sdk if it's not installed (it's a private package)
 if "claude_agent_sdk" not in sys.modules:
diff --git a/backend/tests/test_conftest_db_url.py b/backend/tests/test_conftest_db_url.py
new file mode 100644
index 0000000..4bed76a
--- /dev/null
+++ b/backend/tests/test_conftest_db_url.py
@@ -0,0 +1,18 @@
+"""Regression test for conftest.py's DATABASE_URL handling.
+
+The setup_db fixture drops all tables after every test, so the suite must
+never run against an ambient DATABASE_URL from a developer's shell (which
+could point at a real dev/prod database). conftest.py must force in-memory
+SQLite unless the explicit TEST_DATABASE_URL opt-in is set (as CI does for
+its Postgres service container).
+"""
+
+import os
+
+
+def test_database_url_is_test_url_or_sqlite():
+    """After conftest import, DATABASE_URL is either the explicit
+    TEST_DATABASE_URL opt-in or the in-memory SQLite default — never an
+    ambient value inherited from the shell."""
+    expected = os.environ.get("TEST_DATABASE_URL") or "sqlite+aiosqlite://"
+    assert os.environ["DATABASE_URL"] == expected