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/docker-compose.test.yml

@@ -0,0 +1,35 @@
+# DB-backed pytest lane (R6 S3b): an ephemeral Postgres + the test image, wired together so the
+# `db` fixture (tests/conftest.py) has a real database. The whole thing is throwaway — the pg data
+# lives in tmpfs (RAM) and nothing is persisted. The pure-logic lane still runs WITHOUT this file
+# (plain `docker run … python -m pytest` — DB tests skip when no Postgres is reachable).
+#
+#   docker compose -f backend/docker-compose.test.yml up --build --abort-on-container-exit \
+#     --exit-code-from test
+#   docker compose -f backend/docker-compose.test.yml down -v
+services:
+  testdb:
+    image: postgres:16-alpine
+    environment:
+      POSTGRES_USER: siftlode
+      POSTGRES_PASSWORD: siftlode
+      POSTGRES_DB: siftlode_test
+    tmpfs:
+      - /var/lib/postgresql/data  # ephemeral + fast; the schema is rebuilt per run
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U siftlode -d siftlode_test"]
+      interval: 1s
+      timeout: 3s
+      retries: 30
+  test:
+    build:
+      context: .
+      dockerfile: Dockerfile.test
+    depends_on:
+      testdb:
+        condition: service_healthy
+    environment:
+      DATABASE_URL: postgresql+psycopg://siftlode:siftlode@testdb:5432/siftlode_test
+    volumes:
+      - ./:/app
+    working_dir: /app
+    command: python -m pytest