chore(docker): add proxies_data volume for /proxy endpoint cache

The /photos/{id}/proxy endpoint (added in 1096854) caches transcoded
RAW/HEIC WebPs at /data/proxies/{id}.webp, but the compose file had no
volume mount for that path — files would be lost on every container
restart, forcing repeated full-resolution decodes. Adding a named
volume to both backend and worker so the cache survives restarts.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker-compose.yml

@@ -26,6 +26,7 @@ services:
       - ${PHOTO_DIRS:-./photos}:/photos:rw
       - ~/Pictures:/host/Pictures:ro
       - thumbs_data:/data/thumbs
+      - proxies_data:/data/proxies
       - db_data:/data/db
       - trash_data:/data/trash
     environment:
@@ -51,6 +52,7 @@ services:
       - ${PHOTO_DIRS:-./photos}:/photos:rw
       - ~/Pictures:/host/Pictures:ro
       - thumbs_data:/data/thumbs
+      - proxies_data:/data/proxies
       - db_data:/data/db
       - trash_data:/data/trash
     environment:
@@ -85,6 +87,7 @@ networks:
 
 volumes:
   thumbs_data:
+  proxies_data:
   db_data:
   trash_data:
   redis_data: