fix(video): logo overlays must belong to the workspace; drop debug stderr

- produceVideo checks every logo overlay key against the workspace's
  media_assets, group_assets or branding logo before queueing; a
  well-formed content address alone no longer suffices.
- Remove MEDIA_DEBUG_STDERR=1 from the production compose: it wrote raw
  ffmpeg stderr to an unrotated file, bypassing redaction.

infra/occupymedia/docker-compose.app.yml

@@ -176,7 +176,6 @@ services:
       <<: *appenv
       MEDIA_WORKER_LANE: local
       MEDIA_SCRATCH_DIR: /var/lib/media-worker/produce
-      MEDIA_DEBUG_STDERR: "1"
     volumes: [mediaworkerstate:/var/lib/media-worker]
     depends_on:
       postgres: { condition: service_healthy }