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.
