fix(compose): mount video_cache_data volume on backend + workers

The /playback transcode cache lives at /data/video-cache. That
directory was created in container-local storage (the mkdir at
services/video.py import time), not on a shared volume — so the
worker pretranscode populated its own ephemeral copy while the
backend served from a different empty one, and the cache evaporated
on every container restart.

Add a video_cache_data named volume mounted on backend, worker-light,
and worker-vision so the pretranscoded files actually reach the
serving path and survive deploys.

docker-compose.yml

@@ -50,6 +50,7 @@ services:
       - ${NEXTCLOUD_USERS_HOST_PATH:-./photos}:/nextcloud-users:rw
       - thumbs_data:/data/thumbs
       - proxies_data:/data/proxies
+      - video_cache_data:/data/video-cache
       - db_data:/data/db   # retained so the docker-compose.sqlite.yml override has somewhere to put mulita.db
     # Run Alembic migrations before starting uvicorn. On a fresh Postgres
     # the empty 0001 baseline is a no-op stamp; create_all in init_db then
@@ -152,6 +153,7 @@ services:
       - ${NEXTCLOUD_USERS_HOST_PATH:-./photos}:/nextcloud-users:rw
       - thumbs_data:/data/thumbs
       - proxies_data:/data/proxies
+      - video_cache_data:/data/video-cache
       - db_data:/data/db
       - models_data:/data/models
     environment:
@@ -214,6 +216,7 @@ services:
       - ${NEXTCLOUD_USERS_HOST_PATH:-./photos}:/nextcloud-users:rw
       - thumbs_data:/data/thumbs
       - proxies_data:/data/proxies
+      - video_cache_data:/data/video-cache
       - db_data:/data/db
       - models_data:/data/models
     environment:
@@ -337,6 +340,7 @@ networks:
 volumes:
   thumbs_data:
   proxies_data:
+  video_cache_data:
   db_data:
   redis_data:
   pg_data: