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/docker-compose.test.yml

@@ -29,6 +29,9 @@ services:
         condition: service_healthy
     environment:
       DATABASE_URL: postgresql+psycopg://siftlode:siftlode@testdb:5432/siftlode_test
+      # This lane HAS a Postgres + ffmpeg, so the DB/ffmpeg tests must actually run here: conftest turns
+      # a would-be silent skip into a hard failure (SIFTLODE_TEST_REQUIRE_DB).
+      SIFTLODE_TEST_REQUIRE_DB: "1"
     volumes:
       - ./:/app
     working_dir: /app