test(db): stand up the DB-backed pytest lane (R6 S3b)

The suite was pure-logic only (R2 S2b deferred the DB lane). Add an opt-in `db`
fixture (tests/conftest.py) that hands a test a real session against an ephemeral
Postgres; schema is built with `alembic upgrade head` (the real prod schema needs
the unaccent extension + unaccent_simple TS config that create_all can't provide,
and this also proves the migration chain applies). Each test starts from a TRUNCATEd
clean DB so tests that COMMIT — the whole point, for R6 S2's transaction work —
don't leak.

backend/docker-compose.test.yml wires an ephemeral tmpfs pg + the test image. When
no Postgres is reachable (a plain `docker run`), the DB tests SKIP and the pure-logic
lane is unchanged: 172 passed, 2 skipped. With the DB: 174 passed.

test_db_lane.py smoke-proves commit round-trip + TRUNCATE isolation. The gate wiring
(siftlode.sh) lands in the ops repo.

backend/tests/conftest.py

@@ -0,0 +1,65 @@
+"""Pytest fixtures for the DB-backed integration lane (R6 S3b).
+
+The bulk of the suite is pure-logic and needs no database. This adds an OPT-IN `db` fixture
+that hands a test a real session against a throwaway Postgres (the app's own engine, bound to
+the test `DATABASE_URL`). When no database is reachable — the default `siftlode check` still
+runs the pure-logic tests with no Postgres — any test that asks for `db` is SKIPPED, so the
+pure-logic lane is unaffected.
+
+Schema comes from `alembic upgrade head` — not `create_all` — because the real schema depends on
+things only migrations set up (the `unaccent` extension + the `public.unaccent_simple` text-search
+config the videos.search_vector generated column needs). Building it via migrations also validates
+that the migration chain applies cleanly. Each test then gets a clean database via TRUNCATE so
+tests that COMMIT (the whole point of this lane — transaction-ownership behaviour) don't leak.
+"""
+import pytest
+from alembic import command
+from alembic.config import Config
+from sqlalchemy import text
+
+import app.models  # noqa: F401 — register every model on Base.metadata
+from app.db import Base, SessionLocal, engine
+
+
+def _db_reachable() -> bool:
+    try:
+        with engine.connect() as conn:
+            conn.execute(text("SELECT 1"))
+        return True
+    except Exception:
+        return False
+
+
+_DB_REACHABLE = _db_reachable()
+
+
+@pytest.fixture(scope="session")
+def _schema():
+    if not _DB_REACHABLE:
+        pytest.skip("no test database (set DATABASE_URL to a reachable Postgres)")
+    # Build the real prod schema via migrations (alembic env.py reads settings.database_url, i.e.
+    # the test DATABASE_URL). Idempotent — a re-run against an already-migrated DB is a no-op.
+    command.upgrade(Config("alembic.ini"), "head")
+    yield
+
+
+def _truncate_all():
+    # RESTART IDENTITY so serial PKs are deterministic per test; CASCADE handles FKs. Truncating
+    # BEFORE each test (not after) means every test — including the first, which would otherwise see
+    # migration-seeded rows — starts from the same empty slate and seeds only what it needs.
+    tables = ", ".join(t.name for t in Base.metadata.sorted_tables)
+    with engine.begin() as conn:
+        conn.execute(text(f"TRUNCATE {tables} RESTART IDENTITY CASCADE"))
+
+
+@pytest.fixture
+def db(_schema):
+    """A real session against the test Postgres, on a clean (empty) database. The code under test
+    may commit freely."""
+    _truncate_all()
+    session = SessionLocal()
+    try:
+        yield session
+    finally:
+        session.rollback()
+        session.close()

backend/tests/test_db_lane.py

@@ -0,0 +1,21 @@
+"""Smoke test proving the DB-backed lane works (R6 S3b): a real commit round-trips, and each
+test starts from a clean database (TRUNCATE isolation), which is what lets the transaction-
+ownership tests (R6 S2) trust what they commit."""
+from sqlalchemy import func, select
+
+from app.models import User
+
+
+def test_commit_roundtrips(db):
+    db.add(User(email="lane@example.com", role="admin"))
+    db.commit()
+
+    got = db.execute(select(User).where(User.email == "lane@example.com")).scalar_one()
+    assert got.id is not None
+    assert got.role == "admin"
+    assert got.is_active is True  # server_default applied on flush/commit
+
+
+def test_isolation_starts_clean(db):
+    # If the previous test's committed row leaked, this count would be 1 — TRUNCATE must reset it.
+    assert db.scalar(select(func.count()).select_from(User)) == 0