test(infra): fail-loud DB/ffmpeg lane + client/as_user/make_user fixtures (E1 S2, TEST-4)

The DB-backed lane and the two ffmpeg tests skipped SILENTLY when the DB was
unreachable or ffmpeg was absent, so `siftlode check` could print green while
~60 tests never ran. Make the compose lane authoritative:
- SIFTLODE_TEST_REQUIRE_DB=1 in docker-compose.test.yml; conftest.pytest_configure
  turns a would-be skip (no Postgres, or no ffmpeg) into a hard UsageError.
- install ffmpeg in Dockerfile.test so the thumbnail/poster tests actually run.
- `-rs` in pytest.ini so skip reasons are always visible.

Add the shared TestClient fixtures the risk-core suites build on: `make_user`
(create + commit a login-ready User), `client` (real auth/session stack, no
current_user override — for login/wallet/epoch), and `as_user` (a client
pre-authenticated as a given user, for tests that aren't exercising login).

backend/Dockerfile.test

@@ -8,5 +8,11 @@
 #   docker run --rm -v "<repo>/backend:/app" -w /app siftlode-test:local python -m pytest
 FROM python:3.12-slim
 WORKDIR /app
+# ffmpeg so the thumbnail/poster tests (needs_ffmpeg) actually run in the gate instead of skipping
+# silently — the runtime image already ships ffmpeg, so the test image should too (see conftest's
+# SIFTLODE_TEST_REQUIRE_DB, which now also refuses to run if ffmpeg is missing).
+RUN apt-get update \
+    && apt-get install -y --no-install-recommends ffmpeg \
+    && rm -rf /var/lib/apt/lists/*
 COPY requirements.txt requirements-dev.txt ./
 RUN pip install --no-cache-dir -r requirements.txt -r requirements-dev.txt